Designer Guide

Text size
Always keep in mind that the user has the final say on the text say, as
modern browser can expand or reduce it at will. As a matter of fact, you
should test your website with various browser variations, see how easy it
is break your design... and therefore rework your design in order to avoid
such easy breakage.
That being said, you may start off with a handful of basic text sizes:
 10 to 12px for regular text
 14px for sub-titles
 18px for titles
 etc.
Transparency
Transparent images do not work well with older browsers, particularly with
Internet Explorer, so you should stay away from 24-bits PNG images. As
for GIF images with transparency, use the over a plain background to
ensure they work properly. Avoid 50% opacity by using the same matte
color as the website's background color.
Misc.
 Test your theme with all of PrestaShop's options activated, so as to
see how the theme reacts.
 Edit a product in order to see how the various options influence your
design... if does at all.
 Make sure to have an homogeneous style for your shop.
Usability
We won't delve into complex human–computer interaction theories here,
but rather try to make sure that your shop is accessible to as many
potential customers as possible. Your ultimate goal should be to have
visitors trust your shop enough so that they star buying.
Every time a visitor stumbles upon a usability problem, your site loses
their trust. Ultimately, if the trust reaches a low point, frustration comes
and the visitor leaves your site, without buying anything of course. Such is
the importance of usability in the e-commerce world.
When design your shop's theme, keep in mind that its mission (in addition
to selling goods, of course) is to show your visitors that your website is
managed by serious and competent people.