This may be a more general bug regarding lack of uniformity in the options for panel plugins.

Specifically, the system-load-monitor applet displays on a white background, regardless of the backgrouns color set for the panel. This is visually very annoying as it attracts focus.

On the other hand, battery-monitor and sensor-plugin Dapplets DO display on the background color of the panel.

On the third hand, network-monitor applet displays its title on the background color of the panel, and the graph background on a white background.

Four hands now: The orage panel applet has an option to customize its background color.

It is the system-load-monitor-applet that I personally find annoying. Others can be expected to have other complaints. The underlying issue is inconsistency.