These variables work if you're using SharePoint's default or any other custom master page.
L_Menu_LCID - the LCID of the site you're in. Useful if you need to determine the language of the site. The list of Locale IDs can be found here. I'm using the LCID for localizations in ERTE project. See the example of checking LCID below:
L_Menu_SiteTheme - the name of the theme applied to the site.
There is one more useful variable, but this one can't be used on custom master pages that you created. This one is used in the SharePoint's default pages:
_spUserId - the ID of the logged in user.