Reasons why being able to put a theme on your OS is necessary for it to be accessible
Different people have different accessibility needs; it is impossible to make a single theme that everyone can use
Some accessibility needs are incompatible with others, so having only one or even two "accessibility" modes can't cut it (e.g. dark + high-contrast theme)
The "accessibility" themes are the least tested and most buggy ones; so preventing users from adding themes means we're stuck with the bugs