Preferences - Custom Colors
This tab will only be present if you are using a Starlight CUI Skin. For more information on CUI skins, see also this SL wiki page.
It has a few options which allow you to change the colors of the user interface:
Buttons, Scroll Bars, etc: Set the color for UI elements such a buttons.
Emphasis Text: This is typically the colour that is used for 'loading..' and other interactive text.
Button Label - normal: The colour of the label text on buttons when unselected (except new FUI buttons).
Button Label - selected: The colour of the label text on buttons when selected (except new FUI buttons).
Panel Background - Normal (requires restart): The normal colour of panels and floaters.
Panel Background - Lighter (requires restart): A slightly contrasting colour for elements of panels and floaters so that it is easier to spot lists, title bars, etc.
Two last two color settings are also available in Preferences → Colors -> Miscellaneous:
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.