I usually don’t change my gnome panel background theme, and i just use the default background for gtk themes or i just make it highly transparent to see the rest of the desktop background beneath the gnome panel. I found a really cool collections for gnome panel backgrounds on deviantart designed by half-left looks really good in case you get bored with the default backgrounds and you want to change it.
Most of these gnome panel backgrounds are transparent, it will looks good if you are using dark or bright desktop background.
Install Panel theme background
right click the panel >> properties >> backgrounds >> background image
then select svg file what suits you from the packs you download.