Wrong display of the forum icon in Dark theme

[Claire1817]

In dark theme, the forum icon is not displayed correctly. There is a white block around the icon.
This block is also visible in white theme when the background is not white.
Some screens:

[areox-net]

I created update the icon, which now has no background. Let's test how this works on avatars on both light and dark mode.
I took the opportunity to spec the dark mode of the "Create New Room" page. We could simply use the same dark grey of the image button on the top-left and the default light text for the 3 selection boxes. The other colors should work as they are.