How to change the 3× column blog page to 2× columns?

Technical Q&A
1

Greetings!
I want to change the default themes 3×X Blog page to a 2×X grid page syle. The only options to choose are list or the 3× grid and I can´t find the defining variable in the source code..
Can somebody help?

X X X
X X X
X X X

to

X X
X X

2 
#shopify-section-blog-template .page-width .grid .medium-up--one-third {
	width: 50% !important;

}