Here is the list of special characters:
As for using it, the first column shows code, so to use it in css with before or after create a string from U+25BC to this "\25BC". Here is the example:
If there's no code in this list you can use this converter, if only you know the ASCII code for the character you would like to use. For example, if you want to convert your character ASCII code to usable version for CSS put in the Numeric Value (Decimal) field value like ▼ and click convert. It will display appropriate code in the CSS Value (Hex).
This site will also be useful: http://ikreator.com/special-characters/