Take a tip from me, forget about the fact the LESS is the source of the styling - as you are aware the LESS get compiled into CSS and it is this that you actually see in Firebug. Don't worry about where it came from, instead simply create a CSS statement that will override what you want to change. You do this by copying the elements of the styling your need from firebug and then adding a little more CSS specificity (google CSS specificity to find out how) to make your statements a little "heavier" than those in the compiled CSS (this will force the override). Obviously your statements should be put in a custom CSS file.
This is how to create a custom CSS compatible with Gantry 4...
Simply create a file called "<TEMPLATENAME>-custom.css" and put this file in the CSS folder of the template (where <TEMPLATENAME> is the name of the template e.g. rt_fracture). GANTRY4 will automatically load this CSS file. If you wish, you can also have browser specific files by appending, for example, "-ie9" making the filename "<TEMPLATENAME>-custom-ie9.css"
Perhaps this will help someone, I had a little problem figuring out the actual name of the Afterburner2 template (yeah, I know, I know).
To tweak the CSS in Afterburner2 (Joomla 2.5), create a custom CSS file called rt_afterburner2-custom.css and put it in directory /www/templates/rt_afterburner2/css.