You have to dig into it with a browser using it's developer tools.  All of them have some type of developer tool that let's you investigate the CSS attached to elements and discover which files are responsible for defining each class or id on a page.  You will be looking to override these with your own definitions in your skin's CSS file that match the look and feel you are looking for.   Typically you'd hover over the element you are interested in,  right click,  then find the inspect the element context menu option.  Each browser is slightly different.  Once you get proficient at locating the CSS definitions for elements you can copy those same definitions to your skin's css and easily adjust them to suit your needs.