For LO 25.2 there will probably be a .uno:ChangeTheme on the notebookbar to allow toggling Dark mode on and off - similar to what MSO has in their view ribbon. See bug 160349. I have temporarily used the moon icon in my patch, but it doesn't look great. So if we could get a proper icon that would be great.
The currently used moon is pretty as it but could be drawn a bit more moonish. https://opengrok.libreoffice.org/search?project=core&path=lc_symbolshapes.moon.png
The icon in Office 365 actually have two different appearance: 1. If the paper is dark, the icon is sun 2. If the paper is light, the icon is moon While LibO icon does not support icon switching capability, I can think of some sort of two of them: sun and moon icon
Oh, the uno:DarkMode icon is a toggle type. So using moon is sufficient