<p></p>
<p>I used this solution for dark theme in my app, <a href="https://issviewer.com" rel="nofollow">you can test it here</a>. <a href="https://github.com/pkrasicki/issviewer">Full source here</a>. I think it works pretty well in practice. Perhaps openstreetmap.org could do this too?</p>
<p>With Leaflet it's possible to apply a custom CSS class with the filter just to the map tiles themselves and that's what I did. This way it doesn't affect whatever you draw on the map. One other problem that I had to solve was that map tiles were too bright when they were loading - they had a light gray color.<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/49721209/100754991-9b3d2300-33e3-11eb-8c2c-ae02a990dbd7.png"><img src="https://user-images.githubusercontent.com/49721209/100754991-9b3d2300-33e3-11eb-8c2c-ae02a990dbd7.png" alt="leaflet-map-tiles-loading" style="max-width:100%;"></a></p>
<p>The solution was to simply change the background color of the map element:<br>
<code>background: #1f1f1f !important;</code></p>
<p>There is only one problem left. Since I applied the filter only to map tiles, the map controls still have the default white color. I haven't figured out how to apply a custom class for the controls in Leaflet yet.<br>
<a target="_blank" rel="noopener noreferrer" href="https://user-images.githubusercontent.com/49721209/100755164-ca539480-33e3-11eb-9423-a306a67c32a1.png"><img src="https://user-images.githubusercontent.com/49721209/100755164-ca539480-33e3-11eb-9423-a306a67c32a1.png" alt="leaflet-controls" style="max-width:100%;"></a></p>

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