[chore] Replace `Mutex` with `RwLock` for themes

[cylewitruk]

Since the theme is write-once, read-many, I think a RwLock would be more appropriate than a Mutex. I remember running into deadlocks on accident when writing the multiprogress suggestion PR, and this would simply make it easier to avoid such caveats.

[fadeevab]

@cylewitruk It sounds reasonable, I remember running into deadlocks too, however, due to urgency, I evaded deadlocks via timely unlocking.

[cylewitruk]

@cylewitruk It sounds reasonable, I remember running into deadlocks too, however, due to urgency, I evaded deadlocks via timely unlocking.

Yeah no I get it, I just ran into this with my own theme handling using Mutex and resolved with RwLock, hence I I bring it up here 😊