You can create multiple themes to be associated with your style. These can introduce minor changes such as different fonts faces, colors or background images. In order for this to work it is important that you use all or some of the CSS code from subSilver's overall_header.tpl file. In case you have removed it from your copy of overall_header.tpl, you can simple copy it from subSilver's overall_header.tpl and paste it in your file. You can then remove or edit it as you like. Each of the variable placeholders here takes data from the theme configuration which is done in Administration Panel -> Styles Admin -> Management. The variable names and the field names are more or less the same so figuring out where what goes is relatively easy.

You create the new theme by going Administration Panel -> Styles Admin -> Create. Whats important here is that you choose your template and the CSS stylesheet file you wish to use (just enter its filename). Remember that what you enter on this page will override the CSS in your stylesheet file since it is loaded after it and in order for it to work the <style>...</style> code in overall_header.tpl must be intact (see an unaltered copy of subSilver's overall_header.tpl if unsure).