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/fomantic/_site/globals/site.variables

@@ -2,3 +2,4 @@
 
 @headerFont: var(--fonts-regular);
 @pageFont: var(--fonts-regular);
+@bold: 500;