In the XBMC Wiki
is pretty much all you need to know about XBMC, like how to skin
and more, if you having trouble navigating I suggest using the search function
Make sure before you edit any skin XML's that you have the right resolution folder, './PAL/' is for standard (4x3) resolution and './PAL16x9/' is for widescreen resolution, that matches your settings.
For adding the 2 buttons, you have to edit the Home.xml, you can edit any of the button controls or add them, they are in group id '9000', which is around lines 217-495.
About the custom thumbnail, you have to put the thumbnail in the same folder as the content, and name it to a specific name, I believe it was folder.jpg or it could be default.tbn.
(example, E:\Videos\Naruto\Season 1\folder.jpg)
With custom skins you probably mean themes
, you can find those under the XBMC Live Place, or check out the XBMC Black Market
If you don't find any you like, you can always make your own, you can extract *.xpr files with XPRExpress
and when you're done editing you can compress it with XPRExpress too.
At the moment the site for XPRExpress seems to be under maintenance, you can check the usual places
if it's available over there.
Hope this helped.