Untitled

index.html
==========

<body id=mainpage ...

contact.html
============

<body id=contact ...

default.css
===========

#menu2 .mainpage {
    background: ...
}

#menu2 .contact {
    background: ...
}

#mainpage #menu2 .mainpage {
    background: ...
}

#contact #menu2 .contact {
    background: ...
}