How to change the fonts size in mobile skin?

You may change the size of the font to your own liking.

Font of the messages users send:

#chat { font-size: 20px; }

Username font in the chat area (next to the messages):

.message-user-name { font-size: 20px; }

The username font in the user’s list:

.user-name { font-size: 20px; }

The font in the setting buttons:

#options-mobile li a span { font-size: 20px; }