If you see bugs in your site only in Internet Explorer this information will probably help you.
The common Prestashop 1.5 bug in the Internet Explorer 7, 8 , 9 , 10. This browser allows attaching no more than 31 css files for using. All that exceeds this number shall be ignored by IE. The bug occures in IE6/7/8/9/10, i.e. in all versions that are currently available. Prestashop 1.5 default theme that is crowded with modules may contain more than 40 css files attached. The solution of this situation for a working e-shop is in using css embedded compression functions. Back-office>Preferences>Performance
When developing a theme the modules that have already been prepared and are not going to be used in future should be disabled.
It also should be noted that such browsers as FF, Opera, Safari and Chrome can easily cope with and manage any number of style files.