the formula background does not adjust the template background
I understand what you mean now.
Here are more details on how this works. Each WordPress.com theme has a set of default colors -- they are not pulled from the CSS file. Latex uses those color values if you don't include colors in your Latex formula (see how to set your own colors: http://en.support.wordpress.com/latex/#latex-colors).
For Modularity Lite, the default color scheme has the background color as black (actually #111111 -- a very dark gray) and text color as #eeeeee (very light gray).
To further complicate things, the theme uses a transparent background image for the content background, so the Latex background color will show up as a darker black against it.
So you can either stick with the defaults, or apply your own colors for each Latex formula that you post. If you are using Custom CSS, you can set your background to #111 if you want the Latex to blend in perfectly with your content.
I hope that helps explain it. :)