All you have to do is follow the instructions outlined in this piece by Anna Powell-Smith: How to Make Your Site Look Half-Decent in Half an Hour
The instructions are from 2012 but they still works really well! I took a bunch of the ideas from it to recently jazz up my web site, berniemichalik.com.
You don’t need special tools or deep HTML or CSS skills. Just follow along and you will have a much better looking web site in..well…30 minutes.