Before I continue, you should probably specify your budget explicitly.
With that said, almost anything older than a few years should do what you need to just fine. I have a Lenovo Yoga 710 from 2016 that works decent, and had an old Fujitsu Lifebook from 2010 that wasn’t too shabby as well. Heck, I once booted Linux off a cheap piano black Toshiba laptop originally made for Vista.
Just choose a random old laptop and you’ll most likely be good.
What themes do you have installed, and which theme are you trying to switch to? Also, can you confirm what distro and DE you’re using as well as any major customizations so I can try and replicate the problem in an Arch VM, please?