Actually, I have overriden the Glulx coloring features and the result is that every game which has the default colour scheme shows my colours, and every game that uses extra colours overrides my settings. I like this, because it ensures that I’m seeing what the author intended - and I still get to keep my own colour scheme when the author didn’t set anything. This may be behaviour you find acceptable.
Maybe this is not how Gargoyle works? I don’t know. But the main (dedicated, sort-of-official) Glulx interpreters for Windows, WinGlulxe and WinGit, do work this way. It’s most pleasing.
You can try this out with the Glulx ports of Rod Pike’s “Dracula”. I’ve got my terp quite customised, and yet when I load those games I get treated to old-school colour bliss. Dunno how it’d work with a customised Gargoyle.
I’d just like to point out that you’ve had to customise Gargoyle to ensure the game’s colours display well on Gargoyle, and you’ve had to CSS to ensure the game displays well on web browsers… and made no provisions for any other interpreters, which will therefore definitely not show the colour scheme you intend. Whereas if you’d used the Glulx colouring features (AND supplied the Gargoyle file as some users may have overridden their default colour scheme) the game would look more consistent throughout systems.
Finally, in the end, if your colour scheme is bad on my eyes (yours isn’t, but hypothetically) I’ll do everything I can to change it, and if I can’t, I won’t play the game. So… the player being able to override your colour scheme is A Good Thing (Lazzah would disagree over this - Axe Of Kolt, Adrift version. I ended up disabling fancy colours on my terp for that game because it was hurting my eyes). Stopping the player of doing this by alternative methods is usually self-defeating.
In this case, it’s definitely self-defeating, because you’ve ensured only two interpreters will display the colours you want - Gargoyle and a web browser running Quixe. WinGit, WinGlulxe, iFrotz, the Glulx-running Android apps, Mac terps… those are guaranteed to show the colour scheme you didn’t want.