I wanted a modern & clean theme for MantisBT. Based on https://github.com/Tagirijus/MantisBTDarkTheme
Upload the whole folder into your plugins/ folder in the mantis installation so that you e.g. have MANTIS_INSTALLATION/plugins/MantisBTModernTheme/MantisBTModernTheme.php. After that the plugin should show up on the manage_plugin_page.php page in the mantis settings. There you can simply install it to activate it.
I recommend you to set this colors inside your config/config_inc.php file
$g_status_colors = array( 'new' => '#ffa0a0', # red,
'feedback' => '#ff50a8', # purple
'acknowledged' => '#ffd850', # orange
'confirmed' => '#ffffb0', # yellow
'assigned' => '#c8c8ff', # blue
'resolved' => '#cceedd', # buish-green
'closed' => '#e8e8e8'); # light gray