Right now set_theme() is being called on each frame in the update() loop. Would be better to do this only when the them is initially / subsequently set.

It would be nice if the grid expanded to fit the window when it was scaled up. My vision isn't great, and bigger numbers are always better.

Right now Clippy is pretty unhappy with this code. Most of these warnings are redundant or picky, but all of them could be easily be cleaned up or disabled: there doesn't appear to be anything too tricky there.

The buttons and labels at the top are pretty small. My vision isn't great, and I would prefer something a bit larger.

It would be great to be able to load a puzzle in standard format to play with. Here is a list of some standard formats, most any of which would be fine: https://sudopedia.sudocue.net/index.php/Sudoku_Clipboard_and_File_Formats

This is a big ask, but ideally the solver would give the puzzle a try even if it was incomplete. There are constraint-based algorithms that are floating around that do a reasonable job of solving underconstrained puzzles quickly.

The source code formatting could be improved with cargo fmt.