>>4dm1r4+(OP)
I think providing both a light and dark mode is overkill, lead to odd bugs, and some weird color combinations. A lot of eng time lost here.
>>hartat+r6
Providing both is not a complicated thing, especially with support in CSS, browsers, and frameworks. That is, unless one designs websites that are so complex and buggy already that supporting a second color scheme costs "a lot of engineering time".