Note that the “Online” location is compatible only with the “showing a single variant or a product locations inventory” widget template.
Custom CSS can be specified in the Custom Styles box within the app’s Styles settings. You can access it through Apps > Inventory Widget > Styles.
Once you’re in Styles, scroll down to the bottom of the page to see “Custom Styles.”
The screenshot below shows a custom CSS override that specifies the font size for the location info text.