CSS
accepts a number of color values: hexadecimal colors (e.g. #F1F1F1), RGB colors
(e.g. 255,255,255) and predefined color names (e.g. red). To change the
background color of Scalar pages one needs to begin by declaring the selector
for the entire page (body), the property they wish to modify (background-color)
and then decide how they want to define the color of their choice. To change
the background color of Scalar pages to red using a predefined color name (see
a list of cross-browser redefined color names here) one would insert:

body {background-color:red;}

To
change the background color of Scalar pages to black using hexadecimal colors,
one would insert:

body {background-color:#000000;}

And to change the background color of Scalar pages to bright blue using RGB
colors, one would insert:

body {background-color:rgb(0,0,255);}

Learn more about CSS
background colors here.