A theme cannot style context menu. You could fix this by following the steps below:
Follow the guide at
https://www.userchrome.org/how-create-userchrome-css.html to add a userChrome.css for your Firefox.
Then, copy the code below to your userChrome.css.
menupopup {
--menu-color: #FF92B1 !important;
--menu-background-color: #FFF6F8 !important;
--menu-border-color: #FFE2EA !important;
--menuitem-hover-background-color: #ffe5ec !important;
--menuitem-disabled-hover-background-color: transparent !important;
--menu-disabled-color: #ffcedc !important;
--panel-padding: 4px !important;
}
Finally, restart your Firefox.
