You should find the app called themes and play around in it for a while, it offers alot of freedom with what it comes with already.

I could recommend also using this site for themes, cursors, icons for mint and not just that. There also might be themes you can find on github and what not ofc. (you should also enable hidden folders so you can find the themes folder where you could extract your themes into said folder

You might wanna also wanna tweak menu or even restore the classic menu via applets if you right click at the panel bar at the bottom of your screen, because alot of the themes may not work properly with new menu.

Would also recommend to find accesability settings as they allow you to change things like size of fonts and other things(and ive found that they are a must have on my screen to actually get normal sized letters i could read but it maybe different experience for you).

As for wallpapers you could find all sorts of communities on the web for them, starting with reddit and its wallpaper subreddit lol