Hey! Thanks again for the feedback and the screenshots, they were really helpful.
I just uploaded a new version (v1.1) that fixes the issues you reported. The Darkly theme should be perfectly readable now with the correct contrast, and the program finally remembers your theme preference when you close it.
I also changed the layout to use a resizable splitter, which should fix the weird resizing issue you noticed between the different themes. Let me know if it works better for you now!
If you're still using it, I'd be grateful if you could let me know how your experience is going