To modify any style settings you need to upload a modified stylesheet to CAM using the Advanced customization mode.
With that mode enabled download the current stylesheet applied to CAM and add the following style to the section marked "Status/Error pages" (with your own color value):
#custom .statuspage header,
.errorpage header {
background-color: green;
}
Save the file and upload the modified stylesheet to the Customize Appearance page in the Admin Settings.