Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

less/lib/mixins/border-radius.less

@@ -0,0 +1,18 @@
+// Single side border-radius
+
+.border-top-radius(@radius) {
+  border-top-right-radius: @radius;
+   border-top-left-radius: @radius;
+}
+.border-right-radius(@radius) {
+  border-bottom-right-radius: @radius;
+     border-top-right-radius: @radius;
+}
+.border-bottom-radius(@radius) {
+  border-bottom-right-radius: @radius;
+   border-bottom-left-radius: @radius;
+}
+.border-left-radius(@radius) {
+  border-bottom-left-radius: @radius;
+     border-top-left-radius: @radius;
+}

less/lib/mixins/clearfix.less

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+// Clearfix
+//
+// For modern browsers
+// 1. The space content is one way to avoid an Opera bug when the
+//    contenteditable attribute is included anywhere else in the document.
+//    Otherwise it causes space to appear at the top and bottom of elements
+//    that are clearfixed.
+// 2. The use of `table` rather than `block` is only necessary if using
+//    `:before` to contain the top-margins of child elements.
+//
+// Source: http://nicolasgallagher.com/micro-clearfix-hack/
+
+.clearfix() {
+  &:before,
+  &:after {
+    content: " "; // 1
+    display: table; // 2
+  }
+  &:after {
+    clear: both;
+  }
+}

less/lib/mixins/header-background.less

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+.header-background() {
+  background: fade(@header-bg, 98%);
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  z-index: @zindex-header;
+  border-bottom: 1px solid @control-bg;
+  .translate3d(0, 0, 0);
+  .transition(~"box-shadow 0.2s, -webkit-transform 0.2s");
+
+  @media @phone {
+    height: @header-height-phone;
+  }
+  @media @tablet-up {
+    height: @header-height;
+  }
+}

less/lib/mixins/light-contents.less

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+// This is a mixin which styles components (buttons, inputs, etc.) for use on
+// dark backgrounds.
+.light-contents(@color: #fff, @control-bg: fade(#000, 10%), @control-color: #fff) {
+  &, a, .Button--link, .Search-input {
+    color: @color;
+  }
+  .FormControl {
+    background: @control-bg;
+    border: 0;
+    color: @control-color;
+    .placeholder(fade(@control-color, 80%));
+
+    &:focus {
+      color: @control-color;
+      background: fadein(@control-bg, 5%);
+    }
+  }
+  .Button, .Button:hover {
+    color: @control-color;
+  }
+  .Button {
+    &, &:hover {
+      background: @control-bg;
+    }
+  }
+  .Button--flat {
+    background: transparent;
+  }
+  .Button:active,
+  .Button.active,
+  .Button:focus,
+  .Button.focus,
+  .open > .Dropdown-toggle.Button {
+    background: fadein(@control-bg, 5%);
+    color: @control-color;
+  }
+}

less/lib/mixins/vendor-prefixes.less

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+// Vendor Prefixes
+//
+// All vendor mixins are deprecated as of v3.2.0 due to the introduction of
+// Autoprefixer in our Gruntfile. They will be removed in v4.
+
+// - Animations
+// - Backface visibility
+// - Box shadow
+// - Box sizing
+// - Content columns
+// - Hyphens
+// - Placeholder text
+// - Transformations
+// - Transitions
+// - User Select
+
+
+// Animations
+.animation(@animation) {
+  -webkit-animation: @animation;
+       -o-animation: @animation;
+          animation: @animation;
+}
+.animation-name(@name) {
+  -webkit-animation-name: @name;
+          animation-name: @name;
+}
+.animation-duration(@duration) {
+  -webkit-animation-duration: @duration;
+          animation-duration: @duration;
+}
+.animation-timing-function(@timing-function) {
+  -webkit-animation-timing-function: @timing-function;
+          animation-timing-function: @timing-function;
+}
+.animation-delay(@delay) {
+  -webkit-animation-delay: @delay;
+          animation-delay: @delay;
+}
+.animation-iteration-count(@iteration-count) {
+  -webkit-animation-iteration-count: @iteration-count;
+          animation-iteration-count: @iteration-count;
+}
+.animation-direction(@direction) {
+  -webkit-animation-direction: @direction;
+          animation-direction: @direction;
+}
+.animation-fill-mode(@fill-mode) {
+  -webkit-animation-fill-mode: @fill-mode;
+          animation-fill-mode: @fill-mode;
+}
+
+// Backface visibility
+// Prevent browsers from flickering when using CSS 3D transforms.
+// Default value is `visible`, but can be changed to `hidden`
+
+.backface-visibility(@visibility){
+  -webkit-backface-visibility: @visibility;
+     -moz-backface-visibility: @visibility;
+          backface-visibility: @visibility;
+}
+
+// Drop shadows
+//
+// Note: Deprecated `.box-shadow()` as of v3.1.0 since all of Bootstrap's
+// supported browsers that have box shadow capabilities now support it.
+
+.box-shadow(@shadow) {
+  -webkit-box-shadow: @shadow; // iOS <4.3 & Android <4.1
+          box-shadow: @shadow;
+}
+
+// Box sizing
+.box-sizing(@boxmodel) {
+  -webkit-box-sizing: @boxmodel;
+     -moz-box-sizing: @boxmodel;
+          box-sizing: @boxmodel;
+}
+
+// CSS3 Content Columns
+.content-columns(@column-count; @column-gap: @grid-gutter-width) {
+  -webkit-column-count: @column-count;
+     -moz-column-count: @column-count;
+          column-count: @column-count;
+  -webkit-column-gap: @column-gap;
+     -moz-column-gap: @column-gap;
+          column-gap: @column-gap;
+}
+
+// Optional hyphenation
+.hyphens(@mode: auto) {
+  word-wrap: break-word;
+  -webkit-hyphens: @mode;
+     -moz-hyphens: @mode;
+      -ms-hyphens: @mode; // IE10+
+       -o-hyphens: @mode;
+          hyphens: @mode;
+}
+
+// Placeholder text
+.placeholder(@color) {
+  // Firefox
+  &::-moz-placeholder {
+    color: @color;
+    opacity: 1; // Override Firefox's unusual default opacity; see https://github.com/twbs/bootstrap/pull/11526
+  }
+  &:-ms-input-placeholder { color: @color; } // Internet Explorer 10+
+  &::-webkit-input-placeholder  { color: @color; } // Safari and Chrome
+}
+
+// Transformations
+.scale(@ratio) {
+  -webkit-transform: scale(@ratio);
+      -ms-transform: scale(@ratio); // IE9 only
+       -o-transform: scale(@ratio);
+          transform: scale(@ratio);
+}
+.scale(@ratioX; @ratioY) {
+  -webkit-transform: scale(@ratioX, @ratioY);
+      -ms-transform: scale(@ratioX, @ratioY); // IE9 only
+       -o-transform: scale(@ratioX, @ratioY);
+          transform: scale(@ratioX, @ratioY);
+}
+.scaleX(@ratio) {
+  -webkit-transform: scaleX(@ratio);
+      -ms-transform: scaleX(@ratio); // IE9 only
+       -o-transform: scaleX(@ratio);
+          transform: scaleX(@ratio);
+}
+.scaleY(@ratio) {
+  -webkit-transform: scaleY(@ratio);
+      -ms-transform: scaleY(@ratio); // IE9 only
+       -o-transform: scaleY(@ratio);
+          transform: scaleY(@ratio);
+}
+.skew(@x; @y) {
+  -webkit-transform: skewX(@x) skewY(@y);
+      -ms-transform: skewX(@x) skewY(@y); // See https://github.com/twbs/bootstrap/issues/4885; IE9+
+       -o-transform: skewX(@x) skewY(@y);
+          transform: skewX(@x) skewY(@y);
+}
+.translate(@x; @y) {
+  -webkit-transform: translate(@x, @y);
+      -ms-transform: translate(@x, @y); // IE9 only
+       -o-transform: translate(@x, @y);
+          transform: translate(@x, @y);
+}
+.translate3d(@x; @y; @z) {
+  -webkit-transform: translate3d(@x, @y, @z);
+          transform: translate3d(@x, @y, @z);
+}
+.rotate(@degrees) {
+  -webkit-transform: rotate(@degrees);
+      -ms-transform: rotate(@degrees); // IE9 only
+       -o-transform: rotate(@degrees);
+          transform: rotate(@degrees);
+}
+.rotateX(@degrees) {
+  -webkit-transform: rotateX(@degrees);
+      -ms-transform: rotateX(@degrees); // IE9 only
+       -o-transform: rotateX(@degrees);
+          transform: rotateX(@degrees);
+}
+.rotateY(@degrees) {
+  -webkit-transform: rotateY(@degrees);
+      -ms-transform: rotateY(@degrees); // IE9 only
+       -o-transform: rotateY(@degrees);
+          transform: rotateY(@degrees);
+}
+.perspective(@perspective) {
+  -webkit-perspective: @perspective;
+     -moz-perspective: @perspective;
+          perspective: @perspective;
+}
+.perspective-origin(@perspective) {
+  -webkit-perspective-origin: @perspective;
+     -moz-perspective-origin: @perspective;
+          perspective-origin: @perspective;
+}
+.transform-origin(@origin) {
+  -webkit-transform-origin: @origin;
+     -moz-transform-origin: @origin;
+      -ms-transform-origin: @origin; // IE9 only
+          transform-origin: @origin;
+}
+
+
+// Transitions
+
+.transition(@transition) {
+  -webkit-transition: @transition;
+       -o-transition: @transition;
+          transition: @transition;
+}
+.transition-property(@transition-property) {
+  -webkit-transition-property: @transition-property;
+          transition-property: @transition-property;
+}
+.transition-delay(@transition-delay) {
+  -webkit-transition-delay: @transition-delay;
+          transition-delay: @transition-delay;
+}
+.transition-duration(@transition-duration) {
+  -webkit-transition-duration: @transition-duration;
+          transition-duration: @transition-duration;
+}
+.transition-timing-function(@timing-function) {
+  -webkit-transition-timing-function: @timing-function;
+          transition-timing-function: @timing-function;
+}
+.transition-transform(@transition) {
+  -webkit-transition: -webkit-transform @transition;
+     -moz-transition: -moz-transform @transition;
+       -o-transition: -o-transform @transition;
+          transition: transform @transition;
+}
+
+
+// User select
+// For selecting text on the page
+
+.user-select(@select) {
+  -webkit-user-select: @select;
+     -moz-user-select: @select;
+      -ms-user-select: @select; // IE10+
+          user-select: @select;
+}