What you want to do is open the CSS file for your chosen template (Example: templates/fisubice/fisubice.css). Open that file and look for:
textarea.post
There will be other items on that line, but you want to find the following word in that line:
background
Background will be followed by a colon and a value, this is the hexdecimal color or color name (such as #FFFFFF or white).
You'll want to change the color to a more appropriate color for your site. If background doesn't exist on that line, or if you can't find textarea.post, add the following to the bottom of the CSS file:
textarea.post {background:white;}
Change "white" to whatever color name or hex color value that you want.
Likewise, if you need to change the font color, you would use the following:
textarea.post {color:black;}
Change "black" to whatever color you want. I hope this helps.
For more detailed help on CSS, check out
http://www.w3schools.com/They offer great tutorials on their site.