Rather than sending you back to the (awful) proc template doc, here is a method that always works.
1- List your style components.
proc template;
source styles.sasweb;
run;
2- Spot the line(s) you want to update.
Here, that would be
'docbg' = color_list('bgA');
3- Re-run the code that step one generated, after changing what you want, and create your new style:
3.1) The procedure call becomes
proc template;
define style Styles.Sasweb2;
3.2) The color list has a new color
style color_list
"Colors used in the default style" /
'fgD1' = cx666666
'fgC1' = cxCCCCCC
'fgB1' = cx000000
'bgA1' = cx6495ED
'fgA' = cx003399
'bgA' = cxffffff
'NEWBGCOL'=CXFF0000;
3.3)The color definition for document background becomes
'docbg' = color_list('NEWBGCOL');
Done! Call your custom style from now on.
I know there is a better way, where you call the parent's styles and just overwrite what you need, but the time needed to learn how to use proc template properly vs the very rare occasions when it is needed makes the investment not worth it (in my case at least). I would have thought there would be a code-generating GUI by now to design customs styles.