For some reason version 18.4.0.376.1900 treats DBMS_OUTPUT lines as comments -- same font style, foreground and background colors.
And if you change something in "PL/SQL Syntax Colors - Default Comment" it also changes lines with DBMS_OUTPUT
The other DBMS_ packages are not impacted, only this one.
It looks like a bug....
P.S.
I may be mistaken, but wasn't it a separate syntax color for Oracle system packages in earlier versions?