Wrong directory for default theme's default.css file.

[SugarD-x]

The Theme Manager currently pulls the default.css from the active theme, but if the active theme is the default MantisBT theme, it tries to pull it from a non-existent theme folder called "default" in "themes".

Here is the code:
` /**
* Load the theme file
*/
function loadTheme( $p_event ) {
global $g_css_include_file;
global $g_active_theme;
$themes = getcwd() . "/css/themes/";
$g_active_theme = plugin_config_get('active_theme');

    if (file_exists($themes . $active_theme)) {
        $g_css_include_file = "/css/themes/" . $g_active_theme . "/default.css";
    }
}`

This should probably be updated to detect the default theme when being used instead of a custom one, and pull its default.css properly, either by defaulting to the correct directory, or just not attempting to override it at all. (I'm not sure if the theme manager applies theme CSS files alongside the default one, or instead of it).

I don't know PHP well enough to actually code a fix, but I'm assuming it would be a check in the function as to whether the user had the default theme applied, and if so, use the standard directory instead of an active theme one.