Button styling update for consistency

docs/v2/DEVLOG.md

@@ -594,3 +594,9 @@
 - Updated color definitions for almost all the `input_control` `form` elements.
 - Changed the `placeholder` rules a little bit to work properly for all elements now.
 - Tested a few things in `playground` about `input_control`.
+- Changed a little it of the colors used for `button` components. The `background` color used is custom, not from the material colors of Google Material, as I had to find a middle ground. The only discrepancy so far.
+- Added customizable `$button-group-margin` and `$button-group-box-shadow` for `button-group` to make it more well-defined.
+- Tweaked size variants for `button`s.
+- Added `inverse` `button` style to the list of `button` styles.
+- Removed the `custom` `button` demo that was still lingering in the current flavor.
+- Updated colors and stuff for `button` color variants. They should be consistent now.

docs/v2/playground.html

@@ -75,6 +75,10 @@
     </div>
     <!-- Insert your page content here-->
     <main><div class="container" style="padding-top: 20px;">
+        <button class="small">small</button>
+        <button>normal</button>
+        <button class="large">large</button>
+        <button class="inverse">button</button>
         <p>Use rows and columns for normal alignment, use input-groups in rows without columns for unaligned but just vertical forms or predesigned forms etc.</p>
           <form>
             <fieldset>