Various style fixes (#13372)

- Add alpha variants for primary color
- Make timeline items solid background color
- Fix reaction styles recently regressed
- Fix diff header and make it flexbox
- Numerous smaller fixes for arc green

web_src/less/helpers.less

@@ -1,6 +1,7 @@
 .df { display: flex; }
 .ac { align-items: center; }
 .jc { justify-content: center; }
+.sb { justify-content: space-between; }
 
 .m-0 { margin: 0 !important; }
 .m-1 { margin: .125rem !important; }