Fix AppBar backgroundColor color is lighter than what the theme defin…

[farhanmian]

…es, #41602

Hi, the background color was different because of backgroundImage: linear-gradient, I have removed that and try to fix it.

• [x] I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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[farhanmian]

Hi all, 1 check is failing (argos), could anyone please guide me? what is the issue?