Background:

I am a beginner programmer and I wrote a Sudoku GUI in using winapi32 in C language.

It is currently working and does what it is supposed to do, but because I am still learning, I know it is likely inefficient and could be written much better.

GitHub repo link :https://github.com/reewdgh/sudoku_gui

Please guide me on:
Bug Efficiency Naming anything I could simplify or improve**
Inconsistency

I'd appreciate your feedback on my code. Note: This is not a duplicate, Please refer to this question https://codereview.stackexchange.com/questions/302870/sudoku-gui-in-c-language after this is updated code.