Should be asked in the Flat Awesome area or their website. However if you check the faq on the link:
To edit the text logo simply go to Style Properties -> Flat Awesome Properties -> Edit the text. It's split into two parts to add the color effect. If you want just a plain color remove the text from the Logo Text(Colored) and put your text inside just Logo Text.
You can also edit the color via: Logo Text CSS(left column) or the "Logo Text Color(Second Part)" right beneath where you put your text.
Finally you can also disable the text logo and use an image by unchecking "Use Text Based Logo" and uploading your logo to /styles/flatawesome/xenforo/ then adjust the height via: Style Properties -> Header and Navigation.