Okay, I have two more suggestions, as shown in the image. The red lines indicate areas I think could be improved:
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The header is too wide and takes up too much space. I suggest reducing its height slightly.
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Comparing the right and left sides, the left side is wider and its content is dense, while the right side is narrower. If a colored background is used, it will clearly lose balance, making larger monitors uncomfortable to use.
I suggest using a light gray for the background color, increasing the width of the right sidebar, and adding some expandable function buttons. I found a reference case with a similar style:
Reference: https://mail.google.com/chat