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/_dashboard.less

@@ -61,7 +61,7 @@
     }
 
     .right.stackable.menu > .item.active {
-      color: #d9453d;
+      color: var(--color-red);
     }
   }