Hi Dennis! Thanks for your answer.
Yes, of course I used the search function before I posted and I got more or less all the threads you've linked to as well.
I think my problem is another:
changing the look of the template is not a problem – I wrote my own one anyway; own template image, own css-file.
I want to change the tag's EFFECTS.
For example: I want the tag <COMMENT_POPUP> to type out not only the comment popup link, but the comment popup link AND the numbers of comments too.
Or, to give another example, I want to add a specific css-class to the "a:link" which is generated by the tag <IMAGE_LAST_LINK>.
Do you know what I mean?
I tried that changing the code in the index.php-file because I thought the tag-functions are defined there. I described that it in my first post, but it didn't effect anything.
I hope I made more clearly what I want. Or did I just overlooked something?