It is two designs, not a toggle
Every colour, shadow, border, image and state needs a second definition that works. Shadows in particular stop reading as depth on dark backgrounds, so the visual system needs rethinking rather than inverting.
The case is strongest for prolonged use
Tools people stare at for hours have a genuine comfort argument. A marketing page someone reads for ninety seconds does not, and the effort usually buys more elsewhere.
Dark mode is a comfort feature for software people live in, not a checkbox for a page they visit once.
Half-implemented is worse than none
One section that didn’t get the treatment — a white card, an unreadable badge, an image with a light background — is more damaging than a consistent light design. It reads as broken rather than unsupported.
Respect the system preference
If you build it, follow the operating system setting by default and let an explicit choice override. And define the full palette rather than only the overrides, or the page borrows whatever the host decides — the same discipline as a design system living in tokens.
Contrast still has to pass
Dark palettes frequently fail contrast requirements, especially for muted text and disabled states. Both themes need checking — accessibility is a conversion feature in either one.
A defensible middle path
Commit deliberately to one look, execute it well, and make sure it is readable in bright daylight on a phone. A single excellent theme beats two mediocre ones, and it is far cheaper to maintain — which matters most where a small team owns the site.