Make component themable

[paodb]

This upgrade is necessary for issue FlowingCode/GoogleMapsAddon#159

[paodb]

The Google Maps web component has no dependency on Vaadin. Making it themable using the ThemableMixin would require upgrading the entire component to a specific Vaadin version. This change is not trivial; therefore, addressing the minor marker label styling issue through making it themable is not feasible at the moment, and any update involving the ThemableMixin would need further consideration before proceeding.