Set column width by percent
 

I'd like to set the column width by percent. What works on a 22-in monitor doesn't work on a 15-in one (much less a 7-in one on a phone!). I saw an old (closed) post that said:
You can try adding it with a CSS insert

col.colone {
width: 20%;
}
col.coltwo {
width: 60%;
}
col.colthree {
width: 20%;
}

I see where to put the insert, but can someone explain exactly how to use it? I know how to use a CSS style for text, but not for column width. I'd want it to apply to the entire blog.
Thanks!

A great resource for CSS is http://w3schools.com/css it will give you an explanation of all the CSS options and a chance to test them.

That's for HTML, and I have used CSS quite a bit on my HTML pages. The thing I don't understand is how to implement it in a theme in WordPress.

If someone could explain what I do after creating the CSS insert, that would be appreciated. I'm assuming that I don't have to go to the HTML tab and add CSS styles on every post!

W3schools has an html area and a CSS area

The CSS insert area of the atahualpa theme allows you to override or add CSS selectors and rules to the page being displayed. Once you add a valid selector, the rules will be automatically applied to all elements that match the selectors.

So I would have to put the selector on every post?

It depends. There are ID's and CLASS's already assigned to posts so if you can't use the ones already defined, then yes you probably want to add in some HTML with the class you want