We use custom buttons by using a Library message (of type Look and Feel) for each button. In our case, that allows us to apply translations to the buttons as needed.
Assuming your license does allow for custom library messages, that is what I would recommend leveraging. Basically you create a Library message of type Look and Feel that has your parameters and/or graphics, and then in the Look & Feel of the survey, you can pull that library message in for use. (Edited to add: you'll want to test it in practice and keep it as simple as possible; we have seen HTML formatting apply to the TEXT of the button instead of the layout of the button). So if you are using images you'll want to test that it works as expected.

Thank you! It worked fine...
Just one more question: I was able to define almost everything except for that I do not need a text but have to insert a symbol (a scpecial arrow for which I also have the html code). This actually is only possile using html, right? So something similar to that:
Do you have any idea?
Cheers :-)

I'm not sure of a workaround since it's not a regular character. I think we would probably use a plain "greater than" symbol with or without dashes to "make" an arrow-like approach

(we don't use arrows because we have languages that run right-to-left)

but maybe Qualtrics support would have some suggestions since you've narrowed it down that far?

Arrow as plain keystrokes -->

Thanks for the quick response! But regretfully our company did not like the -> arrows . But Qualtrics support might be able to help :-)