It's not necessarily true, especially if you have a component library and "swappable" themes.
After an upfront investment of setting up the dark and light themes, you can just use your component library and rely on the themes. It's still an investment, but the investment is "constant" and doesn't scale with the number of UI tasks linearly, you don't need to reinvent the wheel, and come up with colors for every feature.
Of course it's a good idea to switch the themes every once in a while, to make sure you didn't miss something, but it's not a huge issue.
It's not necessarily true, especially if you have a component library and "swappable" themes.
After an upfront investment of setting up the dark and light themes, you can just use your component library and rely on the themes. It's still an investment, but the investment is "constant" and doesn't scale with the number of UI tasks linearly, you don't need to reinvent the wheel, and come up with colors for every feature.
Of course it's a good idea to switch the themes every once in a while, to make sure you didn't miss something, but it's not a huge issue.