MDL-69392 theme_classic: Darken red, blue and green colours

The current red, blue, and green colours achieve the minimum colour
contrast against a white background. However, when they're against
coloured background, text with these colours will easily fail the
4.5:1 contrast ratio requirement for Level AA.
The updated colours now have the following contrast ratio against a
white background:
* Blue  - #0f6fc5 (5.12:1, from 4.58:1)
* Green - #357a32 (5.27:1, from 4.63:1)
* Red   - #ca3120 (5.29:1, from 4.59:1)

theme/classic/scss/preset/default.scss

@@ -11,14 +11,14 @@ $gray-800: #343a40 !default;
 $gray-900: #212529 !default;
 $black:    #000 !default;
 
-$blue:    #1177d1 !default;
+$blue:    #0f6fc5 !default;
 $indigo:  #6610f2 !default;
 $purple:  #613d7c !default;
 $pink:    #e83e8c !default;
-$red:     #d43f3a !default;
+$red:     #ca3120 !default;
 $orange:  #f0ad4e !default;
 $yellow:  #ff7518 !default;
-$green:   #398439 !default;
+$green:   #357a32 !default;
 $teal:    #20c997 !default;
 $cyan:    #008196 !default;