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!