Symphony.

How can I change display of font in Symphony, the one which displays, when articles are being written.

Apparently, current font doesn't support display of eastern European letters (čć)

Have a look at Backend Add Script .

Apparently, current font doesn't support display of eastern European letters (čć)

Which operating system are you using? This sounds like bug.

I'm not saying they display strange, but maybe it's the size that's questionable or something, when I enhance text it looks ok, but when letters are small, acute and caron seem to be to big for letters.

I just wonder where could I in original css change font for that?

You can find all the css files in the symphony assets directory. But from 2.4 all the assets are concatenated and minified with the help of Grunt.