Looks neat.
The CtC community is pretty prolific, and if your colors could match theirs that'd be much easier to parse. https://youtu.be/mkomrpQG388?t=212 has most of them all at once, I think palindrome is the only one missing from there.
Also, if your solver could pick up shift for corner marks and control/command for center marks, that'd be a nice quality of life pickup compared to Sudokupad.
+1 on matching the CTC 'standards'. You'll get significantly more user adoption if the conventions match.
Thanks, I was matching colors of another popular Sudoku app. Ultimately it would be best to allow users to configure colors and other presentation attributes (thickness of the line, alpha, dotted/dashed, etc...).
The CtC community sometimes overlaps with, and mentions in their vids, https://www.dailykillersudoku.com/ which also has a nice UX, especially for helping compute totals and possible sum-partitions. Some features might bear imitating/flattering.