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_status_test.go

@@ -114,7 +114,7 @@ func TestPullCreate_EmptyChangesWithCommits(t *testing.T) {
 		resp := session.MakeRequest(t, req, http.StatusOK)
 		doc := NewHTMLParser(t, resp.Body)
 
-		text := strings.TrimSpace(doc.doc.Find(".item.text.green").Text())
-		assert.EqualValues(t, "This pull request can be merged automatically.", text)
+		text := strings.TrimSpace(doc.doc.Find(".merge-section").Text())
+		assert.Contains(t, text, "This pull request can be merged automatically.")
 	})
 }