backup, go to your template and copy everything from the admin menu, color, script in head, script after body, etc. Then,update the template, update the modules(everyone you use), update the php to 8 or higher, then do the J4 update.
I personally copied my whole template for reference as when the new one is updated, it won't be perfect. at least you will be able to look at old css files to see what was done.
Good luck, I am 3 of 6 right now, some minor bugs. but saguaros has been quick to get the issues corrected.
Jim