Seam between space bar and room list can be hard to see, making the expand space bar toggle hard to understand

### Steps to reproduce

Space bar and to lesser extend room list are colored based on the user avatar(?).


In some cases the seam is well visible (lower arrow), in others it is not (upper arrow). In my case that leads to a visibility problem: it is hard to see what the space bar expand toggle is "attached" to. In general I think the app benefits from its different panels being well distinguishable from each other.

This isn't so much of a problem with light mode for me, but still might be for other avatars.


### Suggestion

The avatar based coloring needs to be limited in a way such that there is always enough contrast to distinguish all, but in particular with regard to the expand toggle these, panels.

### Alternatives
Remove avatar based gradient/coloring

### Operating system

arch

### Application version

Element Nightly version: 2021121401 Olm version: 3.2.8

### How did you install the app?

aur

### Homeserver

_No response_

### Will you send logs?

No

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The T-Defect template doesn't fit this very well, but I would still consider this a visual bug so here we are.