Normally, in prestashop 1.5.x theme are in auto detect for ipad and iphone. So prestashop show mobile nad tablet themes in front end. But some guy may not need such features of prestashop. If you want to have same design as in PC you need to do bit workout for it.
1. First of all you need to log in backoffice.
2. Click on Preference > Themes link
3. Now you will get next page of “Themes”. Here, you will get “Enable the mobile theme” section for setting. Here you need to choose “I’d like to disable it, please” and save it.