1.What is the FireMonkey Style Template?

FM Style Template is a template for "Bitmap Style Designer" tool (Tools > Bitmap Style Designer).

For more information about Bitmap Style Designer, please, read documnentation:

http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Bitmap_Style_Designer

2. How can I use template from www.delphistyles.com?

FM Style Template not depends from FM style format and you can use templates since XE5 version -

just run Bitmap Style Designer from your current IDE,

open template and save it as FM style (*.style) with specific for this version style format.

Then you can use *.style files in TStyleBook component.

For more information, please, read documnentation:

http://docwiki.embarcadero.com/Libraries/Tokyo/en/FMX.Controls.TStyleBook

http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Customizing_FireMonkey_Applications_with_Styles

3. Can I edit FM Style Template from www.delphistyles.com?

Yes, you have full rights to edit, rename style template as you want.

To edit and test it, you must use Bitmap Style Designer tool.

For more information, please, read documentation:

http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Bitmap_Style_Designer

Information about how to export / import images from style template:

http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Working_with_Images_in_the_Bitmap_Style_Designer

4. How can I add additional elements to the FM Style Template?

See specific blog about that:

https://community.embarcadero.com/blogs/entry/using-the-bitmap-style-designer-to-create-a-custom-style