In the upper right, you will notice a menu called "PRESET STYLES". For those of you who do not like the dark theme with red text, you have the option to select any of the other 5 board styles and its correspoding detail level.
This is part of your profile and will not affect other user's settings if you want to experiment with it. To get it back to the default, simply scroll down to the very bottom of page and click "Reset Settings" on left.
You also have the option to adjust the text size of the entire board as well (upper right, under the Main menu).