Most people have probably already seen this but in case you missed it in September 2013 Embarcadero released a set of premium styles for Delphi XE5. The styles come in two colors which are Jet and Diamond. There is a style included for each of the supported OSes which are Android, IOS, OSX, and Windows. What I do with my mobile apps is put a TStyleBook on the form, assign it to the form StyleBook property, double click it to bring up the style editor, and then pick the style for the OS that I am currently compiling against. You can also do it via code but for testing purposes this was the fastest way for me. Apparently the Firemonkey styles have support for the different resolutions provided by devices with Retina resolution displays. The style pack should really just come with Delphi XE5 when you install it but maybe it will be integrated with the next version.
FMXExpress.com has over 700 tips and tricks for building cross platform apps on Android, IOS, OSX, Windows, and Linux with a single codebase and single UI using the FireMonkey framework found in Delphi and C++Builder.
FMXExpress covers top cross platform secrets for FireMonkey in C++Builder and Delphi XE5, XE6, XE7, XE8, Seattle, Berlin, Tokyo, Rio, and Sydney.