The bookmark platform has built in media queries so if what you are adding to the page is built responsively, it should not effect the ability for the site to layout on any device. In otherwords, the templates you start with are built mobile first, so they should adapt to whatever screen width. However, when you add things like tables or maps with width in px instead of % widths, be sure to make your code is optimized to widths specified by the device. If you use Chrome Browser, try a Google Chome addon like Emmet Review to check all devices and in order to ensure you don't break something when you add something new
Either way, you'll see the templates already have media queries. When you build your own CSS, you will be overridding theirs.
If you need further help with this let us know.