People pay for convenience every single day. Just look at Starbucks.
If you have a microtransaction for everything in your computer, they start to add up pretty fast. At some point you have to pick and choose, and nice UI themes start to seem less essential...
(Also, do note outside the first world $29 is not such a small amount of money. And $79 is unthinkable for a UI theme).
I do use the free stuff (GitHub, VS Code, etc).
And I also pay for NCrunch, Wallaby, RubyMine, Rider, GoLand, PyCharm, FastMail, PCloud, and more.
Most of the stuff I pay for has free equivalents (and yes, I know JetBrains have an all-products subscription rather than separate Ruby, .Net, Go, and Python, but I have reasons).