Investigate a method to exempt the preferences pane from currently applied contrast theme without requiring an iframe

Currently, when a user applies a contrast theme, the UIO preference pane retains its default theme until the user has dismissed and reopened the pane. This ensures that the pane's content remains readable in case the applied theme is not. Right now, this is implemented using an iframe. It would be good (perhaps alongside a change like that proposed in FLUID-6605) to explore a way of scoping styles to do this without requiring an iframe, which would reduce much complexity. See: https://matrix.to/#/!mDigWuPWilYGiEcOCA:matrix.org/$d8jdfqOPfVSkUQw9mfrmZvWTX_J3_6Q-LmAIoIvjGAk?via=matrix.org