Suggestion: Setting option for transparency colour of user profile icon

[kevinmatthes]

First of all, Happy New Year and thank you very much for your work on this amazing system! 🎇 🎆 🎉

I would like to suggest to allow users to control the transparency colour of their users icons in order to avoid problems concerning the visibility of the image. This suggestion is motivated by my own user icon which is the same for both GitHub and Element, a PNG image with a uniformly black foreground and a transparent background. Element renders this picture with black as transparency colour causing the icon being displayed as an entirely black circle. This problem occurs with both the web client and the Android app.

The simplest solution would be to request altering the default transparency colour from black to some grey tone but this might cause problems for others who did not have any issues regarding their icons' visibility, yet. Thus, a convenient solution for all Element users with transparency in their icons would be leaving the choice of the transparency colour to them. Regarding the implementation, a small palette with at least black (as the default value) and white would suffice for me. For convenience, some grey tones might be added to the palette in a later step, as well, say three equally distributed ones within the range from black to white.