allows user to customise nord airline theme

[dylnmc]

Note: the colors are only sourced one time (the get(..) seems to be evaluated
one time)

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I am willing to modify as much as is desired, but this is just a small push to allow user to be able to customise nord airline theme. I will try to think of a better way, but I will leave this here for now.

[dylnmc]

I will add documentation and pictures later; sorry about that

[arcticicestudio]

Hi @dylnmc 👋
Thanks for your contributions and patience 👍

I've been too busy with my "real" life and the development of the official Nord website to work on the massive amount of issues/PRs that were submitted each day for all of Nord's port projects.

Your changes looking like a nice refactoring and more compact solution, but I'd need to learn more about Vim's scripting engine to see if this could affect performance (e.g. parsing per render or function call overhead instead of constants storing values that can be pushed to the persistent memory on load etc.).
Also there are some other PRs and changes to the airline theme I'd like to merge to deploy them with the next release version. These changes need to be added to yours too since they add some missing or new features of airline like #130 and #134.

I'll move this PR from the backlog to the project board of the next version and will evaluate and test it when I've read some more about Vim's script engine.

[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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@dylnmc Thanks again for your contribution at all! 🙏🏼
This issue has been added to the queue to be picked up to be planned for the next iterations.