Allow setting theme color theme from the menu bar

This functions essentially the same as the language selector. The language selector now also checks the currently set option on launch, and both the language and theme selectors spawn a popup prompting the user to restart NUSGet after selecting a new value.

modules/theme.py

@@ -5,6 +5,9 @@ import os
 import platform
 import subprocess
 
+from modules.config import update_setting
+
+
 def is_dark_theme_windows():
     # This has to be here so that Python doesn't try to import it on non-Windows.
     import winreg
@@ -17,6 +20,7 @@ def is_dark_theme_windows():
     except Exception:
         return False
 
+
 def is_dark_theme_macos():
     # macOS is weird. If the dark theme is on, then `defaults read -g AppleInterfaceStyle` returns "Dark". If the light
     # theme is on, then trying to read this key fails and returns an error instead.
@@ -29,6 +33,7 @@ def is_dark_theme_macos():
     except Exception:
         return False
 
+
 def is_dark_theme_linux():
     try:
         import subprocess
@@ -43,7 +48,14 @@ def is_dark_theme_linux():
     except Exception:
         return False
 
-def is_dark_theme():
+
+def is_dark_theme(config_data: dict):
+    # Theming priority order:
+    #   1. `THEME` environment variable
+    #   2. Theme saved in config.json
+    #   3. The system theme
+    #   4. Light, if all else fails (though personally I'd prefer dark)
+    #
     # First, check for an environment variable overriding the theme, and use that if it exists.
     try:
         if os.environ["THEME"].lower() == "light":
@@ -54,7 +66,18 @@ def is_dark_theme():
             print(f"Unknown theme specified: \"{os.environ['THEME']}\"")
     except KeyError:
         pass
-    # If the theme wasn't overridden, then check the current system theme.
+    # If the theme wasn't overridden, read the user's preference in config.json.
+    try:
+        match config_data["theme"]:
+            case "light":
+                return False
+            case "dark":
+                return True
+            case _:
+                pass
+    except KeyError:
+        pass
+    # If a theme wasn't set (or the theme is "system"), then check for the system theme.
     system = platform.system()
     if system == "Windows":
         return is_dark_theme_windows()
@@ -62,3 +85,16 @@ def is_dark_theme():
         return is_dark_theme_macos()
     else:
         return is_dark_theme_linux()
+
+
+def set_theme(config_data: dict, theme: str) -> None:
+    match theme:
+        case "light":
+            print("setting theme to light")
+            update_setting(config_data, "theme", "light")
+        case "dark":
+            print("setting theme to dark")
+            update_setting(config_data, "theme", "dark")
+        case _:
+            print("setting theme to system (default)")
+            update_setting(config_data, "theme", "")