It is very likely that this background color will be defined in the global configuration file. Go to your site's characters.html file, and confirm which background is being assigned that line. Likely to vary between 2 variables, you should access your server's global config.php, you should find something like:
$ config ['darkborder'] = "# 9ac6e2";
$ config ['lightborder'] = "# bddcf0";
$ config ['vdarkborder'] = "# 475468";
Just change these colors or in the characters.html file you will change the background to a color you want, so instead of assigning 1 color that is defined in 1 global variable you will assign 1 exclusive color to this page.