If you want to use osCommerce, which has much improved over the last few years, be sure to install the 2.3.4 BS Edge version. This is the most up to date release, PHP 5.6 compatible (soon to be 7.0 compatible) and is Responsive (using Bootstrap). It is available on GitHub (instructions are on forums.oscommerce.com). Do NOT under any circumstances attempt to use the latest official release, 2.3.4 ("plain"). It doesn't even run on current PHP levels and has not been updated in years, but is the version most "one button" installers will give you. First check if osC 2.4 has yet officially been released; use that instead of 2.3.4BS if it's out (no idea when it's coming -- as of this writing, it's in early Beta). There's also rumor of a 2.3.5, which is a bug fix version of 2.3.4. It may run on current PHP levels, but will not be responsive.
Up through osC 2.2, the layout was table-driven with very little CSS. Since 2.3, much better use has been made of CSS. It is possible to adjust the CSS to do a great deal of customization. Put your changes in the user.css file, rather than changing stock .css files. That way, whenever you update the code, your CSS customizations won't be lost.