Inherit the background color on spelling error highlights

[chaucerbao]

I have the cursorline option enabled, which highlights the line with the cursor. Lines with spelling errors render like this, because the background is set to a specific color (s:nord0_gui).

This PR simply removes the background color, so it's inherited.

[svengreb]

Thank you for your patience! 🙏🏼
It‘s been a while since I had free time to focus more on Nord, and my open source projects in general, and invest time in this issue due to work-life balance.

I recently published the first “Northern Post — The state and roadmap of Nord“ announcement which includes all details about the plans and future of the Nord project, including the goal of catching up with the backlog. This issue is part of the backlog and therefore I want to triage and process it to get one step closer to a “clean state“. Read the announcement about reaching the “clean“ contribution triage state in Nord‘s discussions for more details about the goal.

Therefore it has been added to the queue in the central and single-source-of-truth project board that is also described in more detail in the roadmap announcement.

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@chaucerbao Thanks for your contribution 🚀
The pull request has been added to the queue and the review will be planned for the next iterations!