Theme: store the last loaded preset as a filepath

This is needed so we can reliably know which extension is associated
with a theme, so a theme can be refreshed on update and cleared when
uninstalled, needed to resolve !120289.

scripts/startup/bl_operators/presets.py

@@ -653,6 +653,8 @@ class SavePresetInterfaceTheme(AddPresetBase, Operator):
             traceback.print_exc()
             return {'CANCELLED'}
 
+        context.preferences.themes[0].filepath = filepath
+
         return {'FINISHED'}
 
     def invoke(self, context, event):

scripts/startup/bl_ui/space_userpref.py

@@ -863,18 +863,34 @@ class USERPREF_MT_interface_theme_presets(Menu):
     def reset_cb(context):
         bpy.ops.preferences.reset_default_theme()
 
+    @staticmethod
+    def post_cb(context, filepath):
+        context.preferences.themes[0].filepath = filepath
+
 
 class USERPREF_PT_theme(ThemePanel, Panel):
     bl_label = "Themes"
     bl_options = {'HIDE_HEADER'}
 
-    def draw(self, _context):
+    def draw(self, context):
+        import os
+
         layout = self.layout
 
         split = layout.split(factor=0.6)
 
         row = split.row(align=True)
-        row.menu("USERPREF_MT_interface_theme_presets", text=USERPREF_MT_interface_theme_presets.bl_label)
+
+        # Unlike most presets (which use the classes bl_label),
+        # themes store the path, use this when set.
+        if filepath := context.preferences.themes[0].filepath:
+            preset_label = bpy.path.display_name(os.path.basename(filepath))
+        else:
+            preset_label = USERPREF_MT_interface_theme_presets.bl_label
+
+        row.menu("USERPREF_MT_interface_theme_presets", text=preset_label)
+        del filepath, preset_label
+
         row.operator("wm.interface_theme_preset_add", text="", icon='ADD')
         row.operator("wm.interface_theme_preset_remove", text="", icon='REMOVE')
         row.operator("wm.interface_theme_preset_save", text="", icon='FILE_TICK')

source/blender/makesdna/DNA_userdef_types.h

@@ -493,6 +493,16 @@ typedef struct bTheme {
 
   /* NOTE: Values after `name` are copied when resetting the default theme. */
 
+  /**
+   * The file-path for the preset that was loaded into this theme.
+   *
+   * This is needed so it's possible to know if updating or removing a theme preset
+   * should apply changes to the current theme.
+   *
+   * #FILE_MAX.
+   */
+  char filepath[1024];
+
   ThemeUI tui;
 
   /**

source/blender/makesrna/intern/rna_userdef.cc

@@ -4301,6 +4301,11 @@ static void rna_def_userdef_themes(BlenderRNA *brna)
   /* XXX: for now putting this in presets is silly - its just Default */
   RNA_def_property_flag(prop, PROP_SKIP_SAVE);
 
+  prop = RNA_def_property(srna, "filepath", PROP_STRING, PROP_FILEPATH);
+  RNA_def_property_string_sdna(prop, nullptr, "filepath");
+  RNA_def_property_ui_text(
+      prop, "File Path", "The path to the preset loaded into this theme (if any)");
+
   prop = RNA_def_property(srna, "theme_area", PROP_ENUM, PROP_NONE);
   RNA_def_property_enum_sdna(prop, nullptr, "active_theme_area");
   RNA_def_property_flag(prop, PROP_SKIP_SAVE);