With versions of Mathematica prior to 10, I was able to add various items to the Format > Background Color and Help menu by creating an init.m in

$UserBaseDirectory/Autoload/PacletManager/Configuration/FrontEnd/

and putting the following code in it:

AddMenuCommands["OpenHelpLink",{Delimiter,
Item["Installed Add-Ons",FrontEndExecute[FrontEnd`FrontEndToken["OpenHelpLink",  {"guide/InstalledAddOns",Automatic}]]],
Item["Standard Extra Packages",FrontEndExecute[FrontEnd`FrontEndToken["OpenHelpLink", {"guide/StandardExtraPackages",Automatic}]]],    
Delimiter}];

AddMenuCommands["BackgroundDialog",{Delimiter,
Item["L&inen",Background->RGBColor[0.980,0.941,0.902]],
Item["A&liceBlue",Background->RGBColor[0.941,0.973,1.0]],
Delimiter,
Item["HelpBrowser Sand",Background->RGBColor[0.964706,0.929412,0.839216]],
Item["SlideShow Slate",Background->RGBColor[0.408011,0.440726,0.8]] }];

This no longer seems to work with Mathematica 10.0.1, even after I restart with a clean cache and clean preferences.

(In case it's relevant, I'm using version-specific front-end preferences, just as I have in previous versions.)