Use substituteAll for fcitx5 theming

fcitx5/default.nix

@@ -1,7 +1,7 @@
 { pkgs, ... }:
 
-let
-  themeFolder = ".local/share/fcitx5/themes/Theme/";
+with import ../theme; let
+  themeFolder = ".local/share/fcitx5/themes/${theme}/";
 in
 {
   i18n.inputMethod = {
@@ -10,9 +10,13 @@ in
   };
 
   home.file = {
-    "${themeFolder}theme.conf".text = import ./theme.nix;
+    "${themeFolder}theme.conf".source = with colors; pkgs.substituteAll ({
+      src = ./theme.conf;
+      background = "${bg1}80";
+      highlight = bg0;
+    } // colors);
     "${themeFolder}arrow.png".source = ./arrow.png;
     "${themeFolder}radio.png".source = ./radio.png;
-    ".config/fcitx5/conf/classicui.conf".text = "Theme=Theme";
+    ".config/fcitx5/conf/classicui.conf".text = "Theme=${theme}";
   };
 }

fcitx5/theme.conf

@@ -1,20 +1,12 @@
-# Modified from https://github.com/tonyfettes/fcitx5-nord
-let
-  theme = import ../theme;
-in with theme.colors; let
-  background = "${bg1}80";
-  highlight = "${bg0}";
-in
-''
 [Metadata]
 ScaleWithDPI=True
 
 [InputPanel]
 Font=Sans 13
-NormalColor=${fg0}
-HighlightCandidateColor=${fg1};
-HighlightColor=${undefined}
-HighlightBackgroundColor=${undefined}
+NormalColor=@fg0@
+HighlightCandidateColor=@fg1@;
+HighlightColor=@undefined@
+HighlightBackgroundColor=@undefined@
 Spacing=3
 
 [InputPanel/TextMargin]
@@ -24,7 +16,7 @@ Top=6
 Bottom=6
 
 [InputPanel/Background]
-Color=${background}
+Color=@background@
 
 [InputPanel/Background/Margin]
 Left=2
@@ -33,7 +25,7 @@ Top=2
 Bottom=2
 
 [InputPanel/Highlight]
-Color=${highlight}
+Color=@highlight@
 
 [InputPanel/Highlight/Margin]
 Left=10
@@ -43,11 +35,11 @@ Bottom=7
 
 [Menu]
 Font=Sans 10
-NormalColor=${fg0}
+NormalColor=@fg0@
 Spacing=3
 
 [Menu/Background]
-Color=${background}
+Color=@background@
 
 [Menu/Background/Margin]
 Left=2
@@ -62,7 +54,7 @@ Top=2
 Bottom=2
 
 [Menu/Highlight]
-Color=${highlight}
+Color=@highlight@
 
 [Menu/Highlight/Margin]
 Left=10
@@ -71,7 +63,7 @@ Top=5
 Bottom=5
 
 [Menu/Separator]
-Color=${highlight}
+Color=@highlight@
 
 [Menu/CheckBox]
 Image=radio.png
@@ -84,4 +76,3 @@ Left=5
 Right=5
 Top=5
 Bottom=5
-''