The WordPress theme customizer is without a doubt one of the most important new features to be added to WordPress, probably ever. If you haven’t used it since WordPress 3.4 came out, you really should.
Daryl Koopersmith was one of the developers who worked on the theme customizer, and he recently released a plugin simply called “CSS”. The plugin adds a box for CSS to the theme customizer panel. So if you want to, while customizing or previewing a theme, you can drop in CSS and watch it update live on your site.
It’s pretty cool, isn’t it? As far as testing out CSS tweaks for a WordPress theme, it’s a pretty slick way to do it that doesn’t involve anything but a browser.
Now, keep in mind that the CSS will, when you save your options, be added to the theme as CSS directly into the header. While you can technically customize your theme this way, it’s not the best way to manage your CSS. What I would recommend, if you use it, would be to test the changes out within the customizer and then copy and paste the CSS that works for you into your theme’s stylesheet.
Aside from the plugin itself, which is cool and you should use it, I would also recommend taking a look at Daryl’s other plugins. I’m actually embarrassed to have never covered any of his other plugins before. I’ll have to get the rest of his worked into Sweet Plugin episodes soon.