There is no reason to make anything other than the dark mode version of my website

FWIW, I get maybe 3 complaints a year that Indie Hackers should have a light mode and the dark is hard to read. I used to get them more often in the early days, but they've largely stopped. I'll probably never have the time or interest to add a light theme.

Here’s a reason: some people find it painful to use dark mode apps/sites. https://blog.discordapp.com/light-theme-redeemed-c541b7ab13e9

A word on accessibility that @Felixg mentions.

Often times is more about the contrast. The ADA compliance establishes a contrast ratio of 4.5:1 or something like that between the text and its background.

So, according to that, you can still have dark theme but the text should match that ratio.

While it's true that you have no reason for supporting a light mode, if you're building this it would be way easier to use variables to define colors and fonts so they can be changed in the future instead of having to rework it.

This is particularly helpful if you're sued by someone because you don't comply with this regulations and you have to do it on a short notice.

Where to put this on your set of priorities depends on your product.

Depends on your product as well. Dark mode is more popular with techies, but maybe not more 'mainstream' consumers. I thought the said thing and just had a dark mode, then got slated for the design (not all of which was due to the dark mode!), so reverted to just light mode for now

I read a book about a marketing specialist that said that we read slower (confirmed by scientific research) on dark backgrounds. It really depends on what you are trying to do. If your user are supposed to 'process' a lot of text then maybe a light theme might also be of interest.

Some people have trouble reading text in dark mode to the point where it becomes painful and unusable for them. It depends on your product of course but if it's text-heavy maybe consider light-mode at least. If you start from scratch you could work with CSS variables to make it easier to accommodate both modes.