Style and template tweaks (#13828)

* Style and template tweaks

- Get red and green buttons on arc green closer to base theme
- EasyMDE adjustments, toolbar and focus border
- Fix header on 404 repo page
- Tweaks to frontpage search, add 'Create Repo' button
- Fix misaligned box headers
- Fix pagination on arc-green
- Fix background and footer on explore and repo search

* better fix for header button alignment

* add label hover for reactions

web_src/less/_chroma.less

@@ -1,7 +1,6 @@
-/* Background */
-
-.chroma {
-  background-color: #ffffff;
+.chroma,
+.lines-code {
+  background-color: var(--color-code-bg);
 }
 /* LineTableTD */