How to change color of text for [A-z]

Hi! I’m a beginner in modifying atom.
I want to change the color of [A-z] characters (alphabet letters) in the editor.
It seems like the color should be specified in styles.less file but I cannot find how to define the set of characters in order to specify the color.
Can anyone help me?

You may wish to develop a grammar for that:

Thanks. I will try to create a grammar.