Why does the number of unread articles decrease, though I haven’t read anything?

Unread articles lose the “fresh” status by Google Reader after 30 days. So if you have unread items which are older than 30 days, it’s a normal and intended feature of Google Reader to mark those articles as read – which results in a decreasing number of unread articles.

How can I adjust the font size of the preview / inline browser?

You can adjust the font size with CMD + (bigger text) or CMD – (smaller text). See “View” menu.

Can you give me a list of all keyboard shortcuts please?

There’s a list of all keyboard shortcuts under “Help” – “Keyboard Shortcuts”.

How can I clear the local cache?

Press and hold the left SHIFT key and start Gruml. Hold the key ’till Gruml has started up, this will delete the local cache.
Or: Inside Gruml, click “Gruml”-”Clear local cache”.

How can I add/remove a feed/tag?

You can add/remove a feed or tag with the “+”/”-” buttons in the bottom bar, or see the “Feed” menu.
If you’ve set Gruml as your default feed reader, any new RSS feed you click on will be added to your Google Reader subscriptions.

How can I add a folder?

Google Reader does not support empty folders (a folder without any subscriptions would be a tag). So you need to add any new folder to at least one of your subscriptions.
To add a new folder, right click on any feed and select “Manage folders” -”Add new folder…”, or use menu “Feed”-”Manage folders”-”Add new folder…”.