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This blog post may help: CSS magic tricks with Embedded Tableau Server views | Tableau and Behold! . To do this, however, you'll need full admin access to Tableau Server and the computer where it's running. Also, as the post states, these changes are not supported by Tableau Support and you'll need to make them after every upgrade.
That said, I was able to modify the background color on the fly on chrome changing this style:
Hope this helps!