Add dark theme to Storage UI

**Is your feature request related to a problem? Please describe.**
Storage UI is targeted at developers and many developers prefer to use dark themes on their dev machines. Currently, Storage UI only has a light theme.

**Describe the solution you'd like**
Add a selectable dark theme to Storage (but default to the light theme like Files UI)

Start by locating the Storage UI theme implementation and compare it with the Files UI behavior referenced in the issue. Add a selectable dark theme while keeping the light theme as the default, and verify that both themes render correctly throughout Storage UI.

Written by the indexing model from the issue text.