stylix(fonts): set default fonts (systemwide)

nixos/configuration.nix

@@ -175,10 +175,14 @@
   fonts = {
     packages = with pkgs; [
       nerdfonts
+      ibm-plex
+      cm_unicode
     ];
     fontconfig = {
       defaultFonts = {
         monospace = [ "VictorMono NF Medium" ];
+        sansSerif = [ "IBM Plex Sans" ];
+        serif = [ "CMU Serif" ];
       };
     };
   };