Motivation for this change

In the past to change colors of this theme you must do

 mkdir -p ~/.themes
 cp -Lr --no-preserve=mode /run/current-system/sw/share/themes/cdetheme ~/.themes/
 chmod +x ~/.themes/cdetheme/scripts/switchtheme
 cd ~/.themes/cdetheme/scripts/
 ./switchtheme ../palettes/Crimson.dp 4 0 0 false true false

Now its enough to do

cde-gtk-theme.override { palette = "Crimson"; numOfColors = 4; }

cc @romildo @NickHu @sauyon

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.