There are quite a few options for further UI customization in System Properties | UI Properties. You can use CSS to change the look and feel of journal fields like Work Notes and the Activity Log, alter the default number of records shown on a list, and configure button styles. It is beyond the scope of this book to go through each and every one, but you can get a good idea of what they do from their description.
Don't forget—you're in your own personal developer instance. You can't break it, so go nuts tweaking things to your heart's content, learning how things work, and generally fiddling. That's what personal developer instances are for! You can always wipe it and re-set it back to factory defaults.