Wiki

You can add this to your custom css if you have access, eg

body#page-mod-wiki-view {background: PowderBlue;}
#page-mod-wiki-view h1 {color: blue;}

You can even target individual wikis, eg.

body.cmid-78 {background: PowderBlue;}
.cmid-78 h1 {color: blue;}
.cmid-78 p {color: red;}

or, you could add it inline in the wiki page if you really have to:

<h1 style="color: blue;">headingtext </h1>