The documentation and use of firemonkey styles is a bit limited. This means it’s a case of figuring out things for oneself or finding other users who have reported useful tips.
The first thing to be aware of is that managing firemonkey styles appears to have changed quite a bit between XE2 and the latest XE6. Most discussion on styles in XE2 is outdated and potentially misleading. I am going to refer here to working with styles in XE6.
The first question is how do we edit firemonkey styles? One way is to use the Bitmap Style Designer (BSD) accessible from the Tools menu. The XE6 documentation suggests that it can edit FMS .style files. However, if you try to load a style using File -> Open the only extension it allows you to open are vsf file, these are style files for the VCL not firemonkey. However I discovered that vsf files are also called Foremonkey Template files. To edit a filemonkey style files you have to work with vsf files and save them as .style files (This is allowed in the Save As menu option). The figure below was taken from the XE6 wiki documentation page and shows a screenshot of the Bitmap Style Designer.
The only way to load a .style file directly is to use the IDE Firemonkey style designer (FSD). To access the FSD you need to right-click over a component and select Edit Custom Style. At the FSD you can then load any .style file you like. The figure below taken from the XE6 documentation wiki shows a screenshot of the FSD.