So over the years the debate always is, should I do a dark or light theme. Some manage to think that a more medium theme is a good compromise although it isn't. High contrast themes offered in both dark and light options is optimal for not only user retention but also for getting new users in the door.
In the 80s there was a study done about reading on a white background with black text and a black background with white text. The tests were overwhelmingly positive for the matter of white with black text is easier to read because of your eye dilation and it makes text clearer. Everyone seems to think that is still the same thing in this day and age. A Dark background doesn't mean "less light" with our technology as it did back in the 80s. Now, with LCDs and LEDs dark backgrounds are just as bright to the eyes as a light or white background. This also doesn't take into account degenerative eye diseases that overstimulated eyes can also be adversely affected by one version or another.
With that said, it's a good move to offer not only a light version of your site (if that's your preferred) and a dark version of your site or vice versa. This gives your users the option of what feels most comfortable to them and provides them with a reason to view the site regularly without that annoying thought in the back of their head of "ugh I hate looking at this site it's too bright/dark".
Of course the question is, well what if I can't make an opposite version of a theme? I don't know code! You can either request help to convert a theme from one to the other or find two themes that compliment one another that you feel will satisfy your need for an alternate theme.
Edited by Morrigan