Would be nice if the logo would respond to different media. Seems like you keep the same logo throughout, which isn’t ideal if you have a larger logo for desktop.
That’s right, but without using php (which is prohibited by phpBB to use within a theme), it’s not possible to change the image.
You can still do this via css (by hiding desktop logo and show mobile logo), but the desktop logo will still load on the mobile site, so you don’t save any website traffic. Also the logo is responsive by default.