Most hardcore developers prefer using a CSS preprocessor over a GUI based tool. For those people we have the SASS Kalatheme project on github. You can check out the installation documentation over there.
Kalatheme makes extensive use of panels layouts to power the structure of most of your content. However, Kalatheme does not ship with any panels layouts by default. As such we highly recommend you make use of Panopoly Theme to get a bunch of great layouts that will work automatically with the Bootstrap responsive grid. To learn more about panels layouts and how to create custom ones check this out.
You might be used to seeing themes with an excessive amount of template files. With Kalatheme we try to keep things simple. What this means in practical terms is you will have most of the static elements for your site like a header in your page.tpl.php file while everything else on your page will be governed with panels layouts... specifically those provided by Panopoly Theme.