I can tell you how to do it, it should be enough to allow you to get started, but some of the more complex topics I'll discuss aren't supported for hand editing, and that is what I'm telling you how to do.
Having said that, here are a few tips on updating the look and feel of your site. First let me advise you should always create a new theme for any site if you will be editing the style in the admin area and saving it as a new theme. This is because it should allow you to update powerstore in the future without overwriting your theme so that you can carry your look and feel from version to version.
Now, when you go to the admin you will be able to select that theme from the admin design tab.
Then, in the folder named after your theme you will see a templates/ directory. It will contain two files: index.php.dwt, and page.php.dwt. (if you used Default it will also contain admin.php.dwt and install.php.dwt. ) Those are the two files that control the look and feel of the front end, site using the basic concepts of Framework builder to change the theme dynamically.
Most of the CSS for theme will be in the /css/[themename]_styles.css, a few form related styles may be located on /css/[themename]_forms.css. This isn't a requirement, it just happens to be what pages the two template pages I mentioned above use. They can be freely modified, moved, or replaced. Some basic structuring may be done in .css in the related /css/[pagename].css file, but those won't typically contain anything that can't be overwritten in the theme css file.
There is also a configurable.css file. This is the css file that is updated by the styler in the admin that allows you to create a new theme from there, so if you use these styles and classes, then when you are finished your new theme would even work when changing colors in the styler.
Diving in a little deeper, you could update the the theme_settings.xml file to add new templates for pages in your site, new css for the edit theme styler to expose, or images to allow update through admin.
Once you are done, take a screen shot of your theme in a browser, and copy over the preview.png file for full integration with the admin.