Thanks for the idea.
Where would be the shade, in your idea ? The menu, or something else ?

I wrote "shade" in but was thinking about a generic color scheme, so you'd have a red / yellow / green color you could choose from, and that would be visible significantly.

That could manifest in having key elements of the UI represent that color, like buttons (I think twice before clicking on a red button than a green one), and/or the top menu bar's color (as seen in #8359).

For those who might want to achieve the same on 4.1.x, where is a howto.

First, pick a color. Each HTML color name which has a "dark*" version is OK, like "red", "green", "blue".

# export COLOR=red

Then fetch the original CSS files and sed then through:

# cd /opt/rudder/share/load-page/files/
# wget http://localhost:8080/rudder/style/{login.css,AdminLTE/AdminLTE.min.css};
# sed -i " 
    s/#f08004/${COLOR}/ig;       # generic menu background color
    s/#e07a07/${COLOR}/ig;       # top left logo background color
    s/#FF8702/dark${COLOR}/ig;   # logo hower
    s/#D47001/dark${COLOR}/ig;   # menu button hower
" login.css AdminLTE.min.css

Create a custom apache config file to override some reverse-proxy settings to serve the modified files

# cat > /opt/rudder/etc/rudder-custom-ssl.conf <<<'
Alias             "/rudder/cache-4.1.5/style/login.css" /opt/rudder/share/load-page/files/login.css
ProxyPass         "/rudder/cache-4.1.5/style/login.css" "!" 
Alias             "/rudder/cache-4.1.5/style/AdminLTE/AdminLTE.min.css" /opt/rudder/share/load-page/files/AdminLTE.min.css
ProxyPass         "/rudder/cache-4.1.5/style/AdminLTE/AdminLTE.min.css" "!" 
';

Just make sure the cache-4.1.5 matches your current version, which you can get from the rudder-server-root package's version.