If the option to create multiple brand themes has been activated for your account, your account's Writer users will notice an additional field - 'brand theme' - is included in the Report Details page for all four report creation methods when creating a report.
Simply select the brand theme required for that report, and all the formatting choices (including logos, colours, fonts and other styling choices) associated with that brand theme will be applied as a "skin" to the report at the point of generating the report.
It is also possible for Writer users to apply a different report theme to an existing report. Simply locate the report in the client's Report Library, click Edit report details, and select another brand theme in the 'Edit report details' window.
On generating the report afresh, the newly selected brand theme will be applied to the report.