DelphiStyles.com sells custom style packs for VCL Windows and FireMonkey multi-device applications. They just released and updated several great custom themes for FireMonkey.
Shown: FMX Profile Screen GUI template (available in GetIt > Sample Projects) with the custom Calypso iOS style applied, available on DelphiStyles.com
Shown: FMX Settings Project sample application (installed with the Samples in 10.2.3) with the custom Calypso Android style applied, available on DelphiStyles.com
Shown: FMX Settings Project sample application (installed with the Samples in 10.2.3) with the custom Malibu Blue iOS style applied, available on DelphiStyles.com
Shown: FMX Settings Project sample application (installed with the Samples in 10.2.3) with the custom Malibu Blue Android style applied, available on DelphiStyles.com
Shown: FMX Settings GUI Template (available in GetIt > Sample Projects) with the custom Malibu Android style applied, available on DelphiStyles.com