[Mashiro]

I'll mention it again . You need to have a look at the CSS of the theme and modify the code there. This can be done in MyBB Administration Control Panel where the styles/themes are. When you click on a style/theme the settings will open and at the top of the settings is a list of CSS files attached to the style/theme. I would look around in global.css and etc to see if you can find the corresponding CSS code for the forum icons. Once you've found it you have to modify it the way you want the icons to be.

You will need knowledge with CSS designs and also how to use webfont icons.