Thank you for the instructions, Pathduck! I've been using the Inspect commands to launch a standalone instance of Vivaldi.
Managed to find the following, which look promising.
.bookmark-folder-picker .vivaldi-tree .cb-form-row .bookmark-folder-picker select { min-width: 100%; } .panel-bookmarks header { display: none; } .panel-bookmarks .panel-bookmarks-edit { display: none; }But it's beyond my CSS skills to do much with them, and I don't know JS. Oh, well, maybe one day.