As of now, there is no such thing as bookmarking a template, but if you've accidentally found the template you like and that you'd like to use later, we'd recommend you do the following:
duplicate the browser tab, change the name of the template to something like “ Ideas for project X”. Close the browser tab- the project will be saved in “My designs” tab and will show up on top of all projects;
In “My designs” click “Add folder”, and change the name to something like “My saved templates” or your project's or client's name. Drag and drop your saved templates into the new folder- you can get back to editing this template later, as soon as you are done with your scheduled posts.