Is your feature request related to a problem? Please describe.
I've created two EUI specific themes for light and dark which Kibana should use as a consumer of EUI. However, the new baseThemeType doesn't take into account a custom theme. Therefore it's still hard to combine a partial theme with a custom theme. Still have to use mergeWithDefaultTheme.

Describe the solution you'd like
Change baseThemeType to be baseTheme which accepts a custom theme object.