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/_tribute.less

@@ -7,7 +7,7 @@
 
 .tribute-container ul {
   margin-top: 0 !important;
-  background: #ffffff !important;
+  background: var(--color-body) !important;
 }
 
 .tribute-container li {
@@ -22,7 +22,7 @@
 
 .tribute-container li.highlight,
 .tribute-container li:hover {
-  background: #2185d0 !important;
+  background: var(--color-primary) !important;
   color: #ffffff !important;
 }