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/_home.less

@@ -25,7 +25,7 @@
     }
 
     .svg {
-      color: #5aa509;
+      color: var(--color-green);
       height: 40px;
       width: 50px;
       vertical-align: bottom;
@@ -45,7 +45,7 @@
   }
 
   a {
-    color: #5aa509;
+    color: var(--color-green);
   }
 }