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.