more… button style

To style it differently, you’d need to use the MoreMoose plugin available from here: https://mookitty.co.uk/devblog/archives/2004/06/27/the-moose-is-loose/
An example of how it works is: https://blog.nuclearmoose.ca/
Explanations are contained within ??
Hope this helps.

Well, sorry, but that plugin changes the style of the text inside the post (text before the [!–more–] “button”). What I need is to change the appearance of the “button” itself.
But thank you anyway…

Put a class on it in the template-functions-post.php file. It’s around line 127 or so.