Gemini Question

[I first posted this in ‘How To’ but it might be better suited to’Design’; apologies for repetition:]
Sorry to post a follow-up/repeat question, but my original never got any answers and I may have worded it confusingly…
Very simply, my Gemini menu shading doesn’t extend all the way down the length of the full page, just the length of the menu text. I have a bottom border to the menu, but removing that doesn’t help: the shading always stops where the text ends, rather than going all the way down the page (obviously the lower boder will be redundant if I get this fixed). When I run Root’s original Gemini css (and the original Gemini index.php) I still get the same problem after adding shading, so I don’t think it’s to do with anything I’ve added since installing it.
Weirdly, the Gemini layout Root is running on the main Gemini page does what I want mine to do: the menu shading runs the full length of the main body. This is exactly what I want mine to do, but I’m stumped.
My .css is at https://www.bwventril.com/weblog/wp-layout.css
Site itself at https://www.bwventril.com/weblog
As ever, thanks in advance (especially to Root, if he has time to help).