In the THEMENAME.skin file, assign a SkinID to the skin:

Then, in the page itself, simply apply the SkinID name to the element you want to have inherit the skin:

Themes can be inherited by both the master page and the content page that uses the master page, and the same SkinID can apply to multiple elements on the same page (for example, if you have three GridViews on the page, all three can have the same SkinID).