Here is his fvwm rc. Given that it's fully documented, I will walk back my assumption that he can barely open a terminal. I researched it a bit and recalled an interview where he said something like "all I use X windows for is is to open a terminal in FVWM", so he clearly can customize it, but he prefers a minimalist setup.
If you scroll down to the bottom of https://cs.stanford.edu/~knuth/programs.html you can see his configurations for Emacs and fvwm and even macOS keyboard layouts; some of them were updated as recently as this year.
This 2020 profile has a photo of him standing at his desk: https://www.quantamagazine.org/computer-scientist-donald-knu... and in the 2008 interview with Binstock (https://mmix.cs.hm.edu/other/knuth-interview.pdf = https://web.archive.org/web/20250408034153/http://www.inform...) he mentioned the set of tools he uses, which includes even “in rare cases, on a Mac with Adobe Photoshop or Illustrator”. Overall he is very comfortable with his computer.
> I designed my own bitmap font for use with Emacs, because I hate the way the ASCII apostrophe and the left open quote […] I prefer rxvt to xterm for terminal input. Since last December, I’ve been using a file backup system called backupfs, which meets my need beautifully […] Incidentally, with Linux I much prefer the keyboard focus that I can get with classic FVWM to the GNOME and KDE environments that other people seem to like better. To each their own.