Photon System for Proton by PV

I tried to replicate the look of the default "system" theme from Photon/Quantum in Proton.

Made with Firefox Color.

Add this code in userchrome.css if you want to keep white context-menus on Windows 10:
menupopup{
--menuitem-hover-background-color: #e0e0e6 !important;
--menu-background-color: #f9f9fb !important;
--menu-color: #15141a !important;
--menuitem-disabled-hover-background-color: rgba(224, 224, 230, 0.4) !important;
--menu-disabled-color: rgba(21, 20, 26, 0.4) !important;
--menu-border-color: #cfcfd8!important;
--menu-icon-opacity: 0.7 !important;
}