Now I am confused. Does this mean that the icons I can use depend on the template? I didn't know that.
Yes. Available icons are always dependent on the template. They aren't a part of "Joomla" per-se. The standard backend admin template and the ProtoStar front end template, which are both Bootstrap v2.1, include the IcoMoon font, and a template.css that defines using those as icons ...
Code:
[class^="icon-"]:before,
[class*=" icon-"]:before {
font-family: 'IcoMoon';
font-style: normal;
speak: none;
}
Other templates include different icons. Some are Bootstrap 2.1 but using Font Awesome or GlyphIcon icons instead of IcoMoon. Some are Bootstrap 3, using Font Awesome, or something else entirely. Some use (say) FA, but with a CSS compatibility layer that allow IcoMoon names to work. Etc etc.
This is one of the reasons it's so damn difficult keeping Fabrik up to date with different templates. For every different combination, we have to come up with the layout overrides for pretty much everything we show on a page. The toughest part is BS v2 vs v3, where pretty much every darn class (especially in the fluid grid handling, popover / popup handling, etc) is radically different.
Handling icons is slightly easier than the main class framework, as all we have to do is have one layout that renders all icons, and does any name substitution needed. So for instance, this is our icon layout for Bootstrap 3 with Font Awesome, which takes IcoMoon icon names used throughout Fabrik, and maps them to the FA equivalent:
https://github.com/Fabrik/Fabrik-Joomla-alt-layouts/blob/master/bootstrap3/fabrik-icon.php
... so when you use that override, most of your IcoMoon icon names will still work.
There are no BS4 templates for J! 3.x, and hopefully never will be, as that would require some massive work. BS4 will happen in J! 4.x.
As for which template to use ... we've had decent luck with most JoomlArt, Gantry 4 and Rocket Themes ones. There are others. Gantry 5 we're more or less working with, although there are still a couple of things to work out.
Basically, make sure they are either Bootstrap 2.1 or 3.x, using IcoMoon or Font Awesome icons, and you should be good. Although you'll have to apply the alternate template overrides:
https://github.com/Fabrik/Fabrik-Joomla-alt-layouts
... and depending which template it is, there may be some tweaks needed.
-- hugh