Ok, here is the decoded text:

{"id":"SteemWorld Settings","version":"1.0","values":{"steem_platform":0,"date_format":2,"refresh_interval":30,"steem_node":"api.steemit.com","desktop_notifications":"all","theme":"dark","coin_site":"coincap","valuta":"USD","image_zoom_factor":0.35,"page_scroll_lists":"allow","reputation_in_name":1,"account_tooltips":1,"account_tooltips_delay":1250,"show_row_gradients":1,"show_title_gradients":1,"show_box_shadows":1,"show_animations":1,"decimal_places":2,"visible_operations":1000,"show_insecure_images":0,"show_amounts_outgoing_votes":1,"recent_posts_view":"default","recent_posts_limit":5,"custom_op_filters":"[]","only_cache_lower_keys":1,"use_keychain":1,"use_steemconnect":0,"show_nsfw":0,"colorize_op_titles":1}}

Looks normal and the setting ( "theme":"dark" ) is in there. Maybe the default settings were not written correctly, before the link was loaded. I will test a few things with different settings again and add a check for detecting invalid 'theme' values.

If it should happen again, please tell me ;)
Thanks!