Note: As of July 9, 2018, Blackbaud’s School Management solutions (which include Core, onBoard, onCampus, onMessage, and onRecord) are utilizing a new top level navigation. This primarily affects users performing administrative tasks—constituent navigation for most parents, students, faculty, and alumni will remain the same. We appreciate your patience as we work to update our documentation. If you are seeing this message, the content of this solution is still being vetted for possible updates. Please refer to this mapping document if you need assistance finding a task in the new navigation.
Yes - @media CSS is available to be used by going to onMessage > Website > Main Site > Styles > Custom CSS.
Your onMessage site already has media queries being styled which can be seen by going here and doing a page search of "media". It is normal for our websites to customize CSS per device (as you will often see in your Custom CSS).