Unfortunately firefox with enhanced tracking protection claims that the user WANTS a light theme instead of indicating no preference - yes, the spec has no no-preference but a) this means the spec is broken because there is no equivalent setting to not implementing the spec and b) web specs are defined by browser vendors and Firefox used to have a no-preference value for this. In general, desktop browsers will also often indicate prefer-color-scheme: light if the user has not actually made an active choice - either as a default in the browser or based on the default system theme. This unfortunately makes prefer-color-scheme: light not actually carry any useful information so there is currently no way to have a dark theme by default but provide a light thome for those users that actually want it (you can only do the reverse).
Unfortunately firefox with enhanced tracking protection claims that the user WANTS a light theme instead of indicating no preference - yes, the spec has no no-preference but a) this means the spec is broken because there is no equivalent setting to not implementing the spec and b) web specs are defined by browser vendors and Firefox used to have a no-preference value for this. In general, desktop browsers will also often indicate prefer-color-scheme: light if the user has not actually made an active choice - either as a default in the browser or based on the default system theme. This unfortunately makes prefer-color-scheme: light not actually carry any useful information so there is currently no way to have a dark theme by default but provide a light thome for those users that actually want it (you can only do the reverse).