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/_user.less

@@ -11,7 +11,7 @@
       }
 
       .header {
-        font-weight: 700;
+        font-weight: 500;
         font-size: 1.3rem;
         margin-top: -.2rem;
         line-height: 1.3rem;
@@ -99,19 +99,19 @@
       font-size: 2em;
 
       &.green {
-        color: #21ba45;
+        color: var(--color-green);
       }
 
       &.red {
-        color: #d01919;
+        color: var(--color-red);
       }
 
       &.purple {
-        color: #a333c8;
+        color: var(--color-purple);
       }
 
       &.blue {
-        color: #2185d0;
+        color: var(--color-blue);
       }
     }