Changing default skin for mediawiki

To change default skin for mediawiki, edit LocalSettings.php file to modify:

$wgDefaultSkin = 'vector';

For example to make default skin as modern use:

$wgDefaultSkin = 'modern';

One can go to skin preferences for a logged in user to see whether modern skin is getting correctly identified as default or not.

Change font size in Modern skin

To change font size in modern skin edit 'skins/Modern/resources/main.css' and update font-size in body appropriately.

body {
...
    font-size: x-small;
...
}