Who I have the same questioned this topic

There are always a lot of apps that start and stop and Android usually does a great job of memory management, usually with no user input.  As far as uninstalling preloaded software, disabling what you can and stopping the sync of any you cannot is about all you can do.  Any Google apps you can stop from syncing from inside your Google Account.  There really aren't many themes per se, and IDK which phone you have, as some might have a slightly different set of backgrounds and icon packs (The LGs do.)