When you do developments for SuiteCRM according to best-practices, they are upgrade-safe. It doens't make sense to stop upgrading when some of these fixes are security fixes, and you should be getting bug fixes and enhancements.
I don't advise attempting to recreate a SuiteCRM Theme. Those are not really themes, they are too intertwined with application logic. SuiteP was a preparatory architectural step towards a new front-end, but only in 8.0 you will see this new front end which I hope has better architecture and allows for more customization.
You can look into tweaking CSS for existing themes, of course. Make sure you start with SuiteP if you want your work to have any future.