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
I assume that most Operating System themes have a "dark mode" icon. My understanding is that LO icon packs are based on different, popular OS themes. So it would then make sense to copy the OS dark mode icon and use it for LO. (That's why I made this bug report - if I'm wrong, then it makes sense to close it.)