Resize sidebar/custom css

Hi,

I like to change the width of my sidebar. I have set up 3 columns, so each one will automatically be 33.3% in width. The 3rd column is the sidebar which I want to be only 25% wide and the two “content columns” to share the other 75%. Is this already possible with the current theme and I only overlook that setting? Sure, I can change the css but than I have to do it after every theme update.

Or, if there isn’t an option for that (and is not planned to implement one): Is there a way for custom css, which will overrule the theme’s css.