You can find some of the mods and themes available in our Client Customization section in our forum. They need to be updated according to the game's current version to work. Note that only PC platforms support themes.
You need to go to the login screen on Mod Management > Import Mod.
You can disable a mod on Mod Management. If you are on Android and the mod uploading failed, you might not be able to upload it again without clearing the app's data. Before doing that, be sure you remember your username and password if you have the Save Password option enabled.
After downloading the theme, place it on PokeMMO > data > themes. Restart the game and go to Settings > Interface > Theme. If installed and decompressed correctly, the new theme will show as an option in this list.
To disable the theme, you can remove it from PokeMMO > data > themes or simply switch the theme on settings to the one you prefer.