To set automatic night mode (dark mode), create a new tiddler and add the $:/tags/PageTamplate tag.
<$vars
currentTime=<<now "hhmm">>
nightStart="2000" <!-- 20:00 -->
nightEnd="800" <!-- 8:00 -->
>
<$list filter="[<currentTime>compare:number:gteq<nightStart>] [<currentTime>compare:number:lt<nightEnd>]">
<style>
html {
filter: invert(1) hue-rotate(180deg);
}
html img {
filter: invert(1) hue-rotate(180deg);
}
</style>
</$list>
</$vars>