Change the sidebar widht of my theme with css

Hello,

i use the Customizr Theme and I only use the right sidebar and the main content. Because of not using the left sidebar the main content shows three posts and that is exactly what i want.

Now, I decided to add a left sidebar for widgets but when i add this sidebar the main container shows only one post and the next post is under it. But I want to have min two or three posts in one row. So i think i have to change the width of the left sidebar but how?

I look in the style.css but there is only the information that i please us customs css to edit something in this style sheet.

can anyone help me? is there a easy css code i can use or where can i find the siderbar width (i know i have to use the child theme)?

Here are some examples how I mean it:

This is exactly how I want the page. The yellow area is where i want my left sidebar and the three posts of my page in the main box.

This is what it looks like when I insert the left sidebar. There is too much space in the yellow area