Ensure color codes are valid at runtime if running in 16-color mode

[Dishti-Oberai]

Zulip terminal runs in 16 color depth mode only if the color codes used in themes for the same are acceptable by urwid. Codes of those 16 acceptable colors are mentioned in the documentation.
In case the contributor uses some other color code instead of the aforementioned codes, the terminal shall not run. The prompted error in such a case, (as of now) doesn't indicate the exact reason for the error.
Construct a test in the codebase to check if the 16 color codes used are acceptable ones and prompt an error otherwise.
PS: This issue is a further development of PR #1154.
Related discussions on CZO: https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/16-color.20names.20in.20themes

[Dishti-Oberai]

@zulipbot add "area: colors/styles/themes"
@zulipbot add "area: tests"

[neiljp]

@Dishti-Oberai Thanks for filing 👍

To clarify, we already have tests via pytest which should detect this in complete included themes, but if for some reason a theme has a modification then upon loading the application itself it may fail. So, this issue is focused on run-time detection of invalid 16-color strings, so I'm removing the tests label.

Arguably there is nothing stopping users from modifying any part of the code and breaking it in whatever way, but themes are more likely to want to be modified, and if we allow external themes at a later stage then we will want such checking in a different form then.

[neiljp]

For anyone taking this up, you could also explore whether we test other color depths for validity based on supported codes.

While I understand urwid itself would not want to test every string passed in at point of use, this may be something to be added upstream in urwid if it is not already there, since it is urwid which defines the valid colors and so is best placed to validate them.

[srdeotarse]

@zulipbot claim

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