Forked off case from @sagebind in #128295 (comment)

I can consistently reproduce this today specifically with terminals in the editor area (similar to #127156) as I experience it every day for the past few months. Here's full reproduction steps:

• Install a fresh copy of VS Code Insiders (no extensions or customizations). At time of writing I reproduced this with commit dc1a6699060423b8c4d2ced736ad70195378fddf.
• Install a theme with a different background color for panel terminal and editor. In this case, https://marketplace.visualstudio.com/items?itemName=swashata.beautiful-ui. Note that this is a theme pack, but can be reproduced with most themes in this pack. The theme shown below is βui - One Dark.
• Create a new terminal in the editor area.
• Ensure that GPU acceleration is on (terminal.integrated.gpuAcceleration).
• Observe the background color discrepancy:

Using the developer tools, you can see that while .editor-container has an inline style of background-color: rgb(38, 42, 49);, the inner element .xterm-viewport has the inline style of background-color: rgb(24, 26, 30);, which does not match the editor area.

Somehow the setting terminal.integrated.gpuAcceleration is actually involved here, because when changed to off, suddenly everything looks correct:

Though the background color of the inner element .xterm-viewport is still the wrong value, it seems to no longer have an impact. I am presuming this is still a VS Code bug, but I suppose that somehow this could be an issue with the theme itself though I'm not sure how.

This test was done on macOS 11.6.