CSS color variables, less bold font weight and more (#13567)

* CSS color variables, less bold font weight

- Define color variables for fully saturated colors and apply them where
  it made sense
- Add background color helper classes
- Globally reduce bold font weight from 700 to 500
- Remove border from timeline icons
- Unify dropzone styling
- Various border style consolidations

* attempt to fix test

* another attempt at tests

* fix contains

web_src/less/features/heatmap.less

@@ -15,7 +15,6 @@
 
   .total-contributions {
     text-align: left;
-    font-weight: 500;
     margin-top: 0;
   }
 }