Really should be like so, for ease of updating later. May seem like overkill but it is the best way. You can skip adding the Your Style part if you wish and are comfortable with updating templates if need be. The merge tool does pretty well.
XenBase
- Apex Light
--- Your Apex-Light
- Apex Dark
--- Your Apex Dark
I initially made Apex dark then Apex light, not sure my reasoning on separating them other than offering them as individual style for sale. Then I decided to just combine them and haven't gotten around to setting one as a child to the other (would take a redesign I'm sure).