Hi Vanessa,
Perphas u can tell us what u are trying to achieve, and we can suggest another way. Coz I think iframe is not make for such function and thus don't have height-100%. Also iframes are by default transparent, pls explain ur 2nd qn.
iframes are for embedding another html file in your webpage. Usually it's a small section and whereby the you want the file to be updated from another file/place
Alternatives: frames, tables...