Added custom themes, import/export option with json (#159)

Reviewed-on: https://codeberg.org/ArkHost/HelixNotes/pulls/159

src-tauri/src/lib.rs

@@ -116,6 +116,10 @@ pub fn run() {
             commands::get_app_config,
             commands::set_theme,
             commands::set_accent_color,
+            commands::save_custom_theme,
+            commands::delete_custom_theme,
+            commands::export_custom_theme,
+            commands::import_custom_themes,
             commands::set_font_size,
             commands::set_font_family,
             commands::set_line_height,