Solution:
You don’t use a WordPress theme by visiting the files under wp-content/themes/ folder. You activate the theme through the admin panel (Appearance > Themes), then the active theme is used through the main website, which you will find under http://www.kwk.de/.
WordPress loads theme files, theme files don’t load WordPress.
You also probably don’t want to use a base tag, as it may conflict with other WordPress code. Instead, use code like the following to access images, or use CSS background images.
<img src="<?php echo get_template_directory_uri(); ?>/img/img.png" />