When the CSS is changed (like with responsive design), the new HTML is loaded.
However the code loads the old css.
This can be easily worked around by adding:

if (file_exists($phpbb_root_path . 'install/update/new/adm/style/admin.css'))

{

	$template->assign_var('U_UPDATED_ADMIN_CSS', $phpbb_root_path . 'install/update/new/adm/style/admin.css');

}

to the install/index.php

But then the images are not referenced correctly, because we use ./ but they do not exist in install/update/new/adm/style/images/ when they were not changed.