SeaMonkey Composer allows you to change the background color of your web page.
To change the background, click on Format menu and choose Page Colors and Background.
In the following window, you may set the web page to use the Reader's default colors or to Use custom colors.
If you want to use custom colors, click on the colored boxes next to the categories (normal text, link text, etc...) of the type of text you wish to change. Select "Background" to change the web page background color.
Then select the color you wish that text or background to be.
Click "OK" and the text you wished to change or background will change colors to the one you selected.