Can I change a generic folder icon to something more representative of what is contained therein?
I'm not sure that has anything to do with Vivaldi.
In Windows Explorer you can open the folder's property and change the type of folder and folder symbol.
If you mean folders on speeddial, then no, you cannot.
I think Guardrail68 means for folders in bookmark manager?
And then it also no you can't. (yet?)