Note though that the edit function doesn't work on smooth entities, only straight or fractal ones, and you must have used a drawing tool to create the original entity as well, as far as I'm aware.

Hydrothermal and other undersea vents come in all shapes and sizes, much as they do on land. As I was working with the deep ocean floor, I wanted something like those relatively recently discovered with entire ecosystems, all with no reliance on sunlight. No plants, naturally, but a fascinating variety of creatures that congregate there. No reason you can't have them just as you've illustrated here, certainly!

It looks to me as if two of the Throne images have been omitted from the set with Temple of Bones Annual, simply judging from their name-labels and the fact the Chair symbol has all four options correctly.

If that can't be resolved, I found some ornate chairs under the Perspectives 3 Bitmap B options that might work (one is varicolor):

Yep, a mapper after my own heart - wanting to make maps complicated, with many different features that can be accessed only by turning layers on or off 😉😁!

Only point I would make is the undersea plants need direct sunlight (or a magical equivalent) to grow, and many tunnels and rooms seem to be entirely roofed-over. Since the water seems to be shallow enough around the island, you could add giant kelp forests in places there (which could help further disguise the undersea access tunnels), as well as other marine plants to help here. They really needn't be just close to the shoreline, as so long as the water's less than about 200 metres deep, there'll be enough sunlight for photosynthetic growth.

One alternative you might try would be not using the RGB Matrix effect, but setting up a simple rectangular polygon that covers all the map on its own, new, sheet, setting the polygon to have a solid red colour, and then adding a Transparency effect to that sheet. If you move the sheet to be below the text, but above the rest of the map sheets, that might work. You'll probably need to keep adjusting the Transparency, and probably also the colour of the polygon (likely somewhere in the reds and oranges range) to get to something you're happy with. Not guaranteed to work ideally, but it should give you more control over the final colour of your text, at least.