I have successfully completed the conversion of the homepage. I've spent a lot of time and effort adapting the Greensteel template to my liking due to lack of experience.

I looked at the „original“ Dokuwiki template, whose functionality is convincing, and took it as a basis with the help of my experiences. I'm amazed at how few changes are required and how the Wiki is preserved in its current form.
Roughly I have

• Added a background image that is fully visible through the semi-transparent content.
• Replaced the logo with the QR code above
• Adjusted the length of the elements to the length of the content (to avoid „bars“ that went over the entire width).
• Provided (almost) all elements with a background color to achieve the contrast of text and background image.
• Made a single change to a PHP script to bring the footer from the bottom of the content to the bottom of the screen. This was necessary due to the background image that should be displayed in full. But the effort of swapping two lines isn't really worth mentioning.
• In addition to the background image, only a new file in the CSS folder was needed to overwrite the existing elements.
• Changed the text, background and link colors in the style.ini.

I plan to publish the newly created template. This requires a version that does not contain any personal data (like the QR code).