Does mailspring support a themes for the tray icon? Many icon themes contain a style for the tray icon:
find /usr/share/icons -name "mailspring-tray*" -print
/usr/share/icons/Papirus-Light/22x22/panel/mailspring-tray-empty.svg
/usr/share/icons/Papirus-Light/22x22/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/Papirus-Light/22x22/panel/mailspring-tray-full.svg
/usr/share/icons/Papirus-Light/24x24/panel/mailspring-tray-empty.svg
/usr/share/icons/Papirus-Light/24x24/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/Papirus-Light/24x24/panel/mailspring-tray-full.svg
/usr/share/icons/Papirus-Light/16x16/panel/mailspring-tray-empty.svg
/usr/share/icons/Papirus-Light/16x16/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/Papirus-Light/16x16/panel/mailspring-tray-full.svg
/usr/share/icons/Papirus/22x22/panel/mailspring-tray-empty.svg
/usr/share/icons/Papirus/22x22/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/Papirus/22x22/panel/mailspring-tray-full.svg
/usr/share/icons/Papirus/24x24/panel/mailspring-tray-empty.svg
/usr/share/icons/Papirus/24x24/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/Papirus/24x24/panel/mailspring-tray-full.svg
/usr/share/icons/Papirus/16x16/panel/mailspring-tray-empty.svg
/usr/share/icons/Papirus/16x16/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/Papirus/16x16/panel/mailspring-tray-full.svg
/usr/share/icons/TelaBudgie/22/panel/mailspring-tray-empty.svg
/usr/share/icons/TelaBudgie/22/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/TelaBudgie/22/panel/mailspring-tray-full.svg
/usr/share/icons/TelaBudgie/24/panel/mailspring-tray-empty.svg
/usr/share/icons/TelaBudgie/24/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/TelaBudgie/24/panel/mailspring-tray-full.svg
/usr/share/icons/TelaBudgie/16/panel/mailspring-tray-empty.svg
/usr/share/icons/TelaBudgie/16/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/TelaBudgie/16/panel/mailspring-tray-full.svg
/usr/share/icons/ePapirus/24x24/panel/mailspring-tray-empty.svg
/usr/share/icons/ePapirus/24x24/panel/mailspring-tray-full-newitems.svg
/usr/share/icons/ePapirus/24x24/panel/mailspring-tray-full.svg
But in the tray, the original icon is displayed