Change width of content in Theme Colinear

Hi,

i am trying to Change the widh of the contet of my page. I am using the Colinear-Theme.

Page: https://www.salvenmoser.org/wordpress

All what i want is that the Content has more width.
I was trying already to install a Custom CSS and enter there the new with, but it didn’t work.

Can somebody help me?
I am very new in CSS, i was already reading how it should work but i can’t make it…