On occasion you may wish to override the theme choice members have made (for example, when you install a new default theme). The IPS Community Suite allows you to do this easily on a per-group basis.
Navigate to Customization -> Themes in the AdminCP, and click the dropdown arrow next to the theme you want to force as the selected theme. Select Set as Members' Theme from the menu:
In the popup window that will appear, you can choose which groups (or all of them) to reset. For example, if a member in the Members group had chosen the Green Theme to use, by using the Set As Members' Theme and selecting the Members group on the Red Theme, that member's choice will be reset and they'll see the Red Theme in future.