You can apply rem-level formatting like header and highlight by using the bottom toolbar when you have multiple rem selected.
I’m not sure if it makes semantically sense to have bulk operations for inline formatting like bold/italic/underline since these do not apply formatting to the rem as a whole, but only to (a part of) the content of a rem.
I think it would be better to use styling tags for that as well. Like
[data-rem-tags~="bold"] .rem-text { font-weight: bold; }
And now you can apply these styling tag to multiple rem selected rem by copying the styling tag with Ctrl + Shift + S first and paste it onto the selection with Ctrl + V. The only thing missing for this workflow is a builtin way to associate a specific tag directly with a shortcut. You can use any third party text expander capable of pasting text though, e.g. Autohotkey.