What is the best way to alter the CSS in a WordPress theme? I’ve been BMHOK for hours to add a gutter to a grid. My BP is currently 166/106. Theme and BP pic in comments.

[The theme is Studio 8]) and I can’t for the ducking life figure our how to add a a column and row gutter in the damn image grid.

I am using vscode to alter the style.css and “adding in additional” css classes. but I can’t seem to affect the grid in the way I want to which is like 12px margin/padding. I know it is probably something dumb like it’s in the actual PHP file.

***ANY*** help is appreciated. TIA Also, here is [BP pic])