Please help customizing theme

I'm trying to customize a new theme changing default values for example buttons and input elements or forms if you like. What I have discovered so far is that the _custom-variables.scss file only changes values made by MDB, not an original bootstrap. So if I'll put there $border-radius-base: 0, buttons and some other elements are getting this value except the form elements, eg input, select etc...

After hours of digging inside all files, I have copied bootstrap scss files in root scss folder of mdb, and included seperate custom.scss file at the beginning for bootstrap itself, and now $border-radius variable is working. But... it seems I'm doing it wrong anyways. There must be some other, official approach of doing this.

_custom_variables.scss (for buttons etc.) $border-radius-base: 0;

_custom_variables-bs.scss (for bootstrap forms) $border-radius: 0; $border-radius-lg: 0; $border-radius-sm: 0;

Is there way to change original bootstrap values without copying these files?

Any ideas?