I've used a lot of themes, and the "customization" was usually limited. Brimstone has such well-designed and well-thought out customization that very little css (actually less) tweaking was needed.

I wanted to use an add-on with this theme, and it just wasn't compatible. James made a major revision in the Brimstone theme to accommodate, and resolved the conflict over the weekend! This kind of support is beyond even what I've found in the high standards in the passionate C5 community!

Awesome theme, and amazing support.