Currently, there is no formal custom dark theme background support in Mobile. Changing sheet background color to black does not work, as this is controlled by a native Android/iOS component instead of CSS. You could use the device's dark mode and still use Qlik Sense Mobile SaaS, however we don't support it. It is possible to apply other theme colors using the JSON properties, but results may vary compared to Cloud SaaS. Any customization documentation would be on the qlik.dev site for SaaS, as Mobile SaaS falls under the SaaS umbrella of support.
Please note that there are appearance enhancements planned in the December Mobile SaaS release, so it is possible that support for custom themes will improve.