Since [QGIS 3.22](https://qgis.org/en/site/forusers/visualchangelog322/index.html#feature-invert-colors-filter-option-for-raster-rendering), there is the option to '*Invert colors*' in raster layer symbology. 

It's not perfect, but a quick way to get started with a Night Mode could be to check the `Invert colors` option on the OpenStreetMap layer. Play around with the `Gamma` and `Colorize` options to tweak the colours.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/TECGV.jpg