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This feature is already available. The current version of the mobile app uses the Portal styling settings, including the logo.
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You can change the logo (displayed in the top-left part of the portal) in the settings of themes used in the system.
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I've verified before my previous answer and, as much as I hate to say it, it worked for me. Therefore, some more in-depth investigation may be needed. I recommend contacting our support (support.webcon.com) so they could help you individually.