I wanted a dark theme for MantisBT. While it's not enough to just add a default.css into the config_inc.php variable $g_css_include_file, I tried to write a plugin, which replaces the needed CSS classes. Maybe there are still some ugly colors or things - I'm no pro here. (-;
Upload the whole folder into your plugins/ folder in the mantis installation so that you e.g. have MANTIS_INSTALLATION/plugins/MantisBTDarkTheme/MantisBTDarkTheme.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.
Basically you could now add your own themes by copying the dark_theme_....sass and compile it to a CSS. Assign the variable $g_dark_theme_options in the plugins config.php like so:
$g_dark_theme_options = [
'tagi_default' => 'Tagi (default)',
'tagi_darker' => 'Tagi (darker)',
'orange' => 'Orange',
'ocean' => 'Ocean',
'your_name_here' => 'You readable style name'
];ATTENTION: next plugin update will overwrite this all again ... maybe I will come up with a more dynamic solution at some point. It's rather hard structured to me ... /-:
And the style sheet file should follow this pattern: dark_theme_NAME.sass|css. Feel free to ask. Maybe I can help.
You can view the changelog here: CHANGELOG.md.