So, the look of say, a program like Hydrogen, what’s that called? A skin? I can change them, correct? Like even the way my whole Ubuntu looks?
Yes, look and feel can be customised. This can be automated, and there a great many distributions and desktop managers floating on top of Ubuntu.
You could head over to DistroWatch and check some out. I personally like "kicker" task bars, and it is possible to dress Ubuntu so that it feels like a Mac.
Wallpaper, screensavers, window dressing, it's all eye candy. Sooner or later you'll need to get down to work and it should not get in your way or be a distraction. It's OK to feel proud.
as mckenzm said, yes you can. May I humbly recommend communitheme ?
with your trusty terminal :
sudo add-apt-repository ppa:communitheme/ppa && sudo apt update && sudo apt install ubuntu-communitheme-session
then open GNOME Tweaks, and configure it like so :
then turn off autologin if you have it on, then reboot
choose this on login :
and now you can turn autologin back on if you had it on.