I really dislike transparent panels. I am using the "breeze" theme in KDE5 / Plasma.
I found the theme folder, and below that was a compressed SVG for the panel background.
It is possible to edit that file using Inkscape. But since it's owned by root, you may want to assume ownership first.
theme_file="/usr/share/plasma/desktoptheme/breath/widgets/panel-background.svgz"
sudo chown $UID "$theme_file"
inkscape "$theme_file" &
But in my case, I decided to write a script to apply the changes (now and in future):
#!/usr/bin/env bash
set -e
theme_file="/usr/share/plasma/desktoptheme/breath/widgets/panel-background.svgz"
if [ ! -e "${theme_file}.ORIGINAL" ]
then sudo cp -a "$theme_file" "${theme_file}.ORIGINAL"
fi
cat "${theme_file}.ORIGINAL" |
gunzip -c |
# Remove transparency from main panels by changing opacity to 1.0
sed 's+fill:#292f34;fill-opacity:0.86274511+fill:#292f34;fill-opacity:1.0+g' |
gzip -c |
sudo tee "$theme_file" >/dev/null