How to change the color of the add new thread button (control_content_background)

Style Manager for your style, go to Stylevars, then control_content_background, in the
Common section

Then…

Background Color = something that matches your image color

Example, the URL to the image.

[CODE]url(path/to/image/test.jpg)[/CODE]

Background repeat = repeat-x (in my case)
units = px (in my case)
Background Image Horizontal Offset = 0 (in my case)
Background Image Vertical Offset = 0 (in my case)

You have to set all these variables or the image wont show.