Im still new and leaning but I think it is under elgg/views/default and under the css file.  If you have a theme it will over ride that. 

Use firebug and make the changes on css.php files.

I already checked css.php and page_elements, but it seems I can't find it, I want somebody to give me a specific place.

I think I should add background: #color under some sections, but I don't know where.

Take a look at the default css file in views/default/css you'll find colors there to change on the basic layout, for instance Recent Members is found in #two_column_left_sidebar_boxes .sidebarBox

it's recommended to make your own plugin or theme and override the css file with your own modified one.

That CSS class could also reside inside /mod/custom_index/views/default/custom_index/css.php/

Thanks Shouvik Mukherjee! yes it was there in custom_index.