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

integrations/pull_merge_test.go

@@ -194,7 +194,7 @@ func TestCantMergeWorkInProgress(t *testing.T) {
 		req := NewRequest(t, "GET", resp.Header().Get("Location"))
 		resp = session.MakeRequest(t, req, http.StatusOK)
 		htmlDoc := NewHTMLParser(t, resp.Body)
-		text := strings.TrimSpace(htmlDoc.doc.Find(".attached.merge-section.no-header > .text.grey").Last().Text())
+		text := strings.TrimSpace(htmlDoc.doc.Find(".merge-section > .item").Last().Text())
 		assert.NotEmpty(t, text, "Can't find WIP text")
 
 		// remove <strong /> from lang