If your theme is a block theme with a dark style of its own (Twenty Twenty-Five has Evening, Twilight, Sunrise and Midnight), choose Your theme's own dark style under Colours → Darkening Method and pick the style. Dark mode then uses that style's colours instead of recolouring the page: your theme's palette, the page, links, buttons and your theme's section styles take the colours the style gives them, and nothing extra is downloaded. Only the colours change, not the style's fonts or spacing. The Dashboard suggests it when your theme has one.
Only styles that stay readable once applied are offered. If one of your theme's dark styles is missing from the list, some part of it, such as the text of a section style, reads below 4.5:1 in it.
A colour typed into a block by hand, rather than picked from your theme's colours, stays as it is under this method. Pick a theme colour for it in the editor, or add a rule under Advanced → Custom CSS.
The choice is not offered on a classic theme. If you change theme, pages are darkened with Automatic until you choose one of the new theme's styles. On a theme with no dark style, saving the Colours screen keeps the style you chose, so switching back to its theme uses it again. To use another method there instead, select it and save.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
