Frontpage and Heatmap CSS tweaks (#13443)

* Frontpage and Heatmap CSS tweaks

- Make heatmap use primary color
- Defined secondary color shades
- Set various blue colors to CSS vars
- Misc tweaks

* remove a useless variable

* remove another useless variable

Co-authored-by: Lauris BH <lauris@nix.lv>

web_src/less/index.less

@@ -2,6 +2,7 @@
 
 @import "./features/gitgraph.less";
 @import "./features/animations.less";
+@import "./features/heatmap.less";
 @import "./markdown/mermaid.less";
 
 @import "_svg";