ETA: My troubleshooting skills still had a bit of rust on them, because I only just thought to try looking at the wiki logged out. Lo and behold, it looks fine, so somehow, there is something wrong with my personal account settings. O.o
Original message follows:
I got the "recommended dimensions" for the header here: https://community.fandom.com/wiki/Help:Background
I brushed the dust off my troubleshooting skills and had a look at my wiki after turning off AdBlock (no change), on a couple different browsers (Chrome and Firefox), and on my cell (Android). The background image doesn't display at all on mobile, but it looks equally stretched out on both my browsers. I use Chrome by default.
If changing the image and/or CSS will fix it, that's easy enough. Thanks for providing the code!
I actually did have the icons vertically centered within the image to begin with, but I lowered them a little in an attempt to keep them from being cut off at the top. I'll happily put them back in the center. This likely isn't a final design, anyway; just working with something similar to the background we had before while I get to grips with the new layout and such.
Truth is, I don't mind an excuse to refresh my wiki's design. I just want the tools I'm given to work so I can design with confidence. {= )
If the devs are watching: There used to be a way to preview articles at different resolutions. It would be nice if that existed for the Theme Designer, since the point about no one perfect resolution is very valid!