I agree that FireBug is a great tool. I too use it daily.
However, it is not helping me make the tweaks in the size of the category bars height and the font sizes as the person who started this tread asked about.
When I use FireBug to find the element for the category bar. this is what it shows:

As you can see it is referring to ccss.php line 163, but there is no such element in this file. I can use FireBug, as you suggested, to edit this size and see what it looks like, however, I can't find the actual file to change.
In the last version I added "height: 30px" to the element "div.nbCategoryHeader" as follows:
div.nbCategoryHeader {
height: 30px;
border-top-style:solid;
border-top-width:1px;
padding:4px 12px 0;
position: relative;
}
This worked back then, but when I add this to the current css it does nothing.
So I used TotalCommander (another must have file management program) and searched all the documents within styles/ninjaboard_blue folder for "45px" and the only files it is in are: font-size.1.0.css, font-size.2.0.css, and .conditional.css.swp though I can not see anything inside this last one.
And I found where to change the bars, fonts and slider in the font-size.1.0.css.
This was a long way to go to figure this out. I really hope you are able to write down some basic instructions for doing these tweaks in the future.