User colors in threads

[SpiritCroc]

While ideally, threads would get some bubble treatment as described in https://github.com/SchildiChat/schildichat-desktop/issues/126, a first step to make them look Schildi-native would be updating some colors:

• [x] Currently, when you hover a message, you get an element-green bar on the left. We should replace that bar with schildi-green, or remove it completely to match the main view.
• [ ] User colors should follow the setting for power-level-based / mxid-based coloring.

[github-actions[bot]]

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