Add support for SDDM about stylix

[name-snrl]

There is a simple sddm theme that you can use

  environment.systemPackages = let bg = "#2e3440"; in [
    (pkgs.where-is-my-sddm-theme.override {
      themeConfig.General = {
        background = "${pkgs.nixos-icons}/share/icons/hicolor/scalable/apps/nix-snowflake.svg";
        backgroundFill = bg;
        backgroundMode = "none";
      };
    })
  ];

[NovaViper]

🤔 I actually think we could use the services.xserver.displayManager.sddm.settings and services.xserver.displayManager.sddm.theme option for configuring the SDDM theme. I actually made something like below a while back when I was using a custom module for setting the cursor themes across the system

       services.xserver.displayManager.sddm.settings.Theme = (mkMerge [
          #({ Font = "Noto Sans,10,-1,0,50,0,0,0,0,0"; })
          (mkIf (cfg.cursorTheme != null) {
            CursorTheme = cfg.cursorTheme.name;
            CursorSize = cfg.cursorTheme.size;
          })

Which works for changing the cursor theme and size for the stock Breeze them; haven't tested using other themes however. There's also the ArchLinux wiki that describes what theme settings are available for SDDM, so we could also use this as a baseline for creating a module to modify SDDM. Would this be something possible to do @danth?

[trueNAHO]

Potentially related:

• #297