css - Customizing a WordPress theme without changing it? in WordPress

While you can modify it without changing it, you can isolate the parts you change by creating a Child Theme. In summary:

1. Create a theme directory on peer with your "parent" theme,
2. Create a style.css file in your new directory that has a Template: declaration in the comments naming your parent theme and an @import url(../%parent-theme%/style.css) to import the CSS from the parent theme,
3. Activate your new theme in the WordPress admin console,
4. Add new files and/or copy files from your parent theme directory to your child theme directory and modify them to your preference, and
5. That's it!

I could give you lots more details but basically this guy does a really good job of explaining How to Create a Child Theme so better for me just to point you to it.

When you want to upgrade the parent theme just upgrade; it will leave your child theme in-tact. Of course your child theme mayt work perfectly if they've changed the parent too much and/or if you copied and modified theme files they updated in the new version you won get the new functionality without modifying them too, but it's a lot better from starting over each time!

Hope that helps.