Fix #4012 : Renaming all colors in colors.xml with correct naming conventions.

[ayush0402]

Explanation

Fixes #4012

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[ayush0402]

Removed oppiaPrimaryDark and renamed all its usage to oppia_primary(previously colorPrimary) as it was re-declaration for same color code.
Same for navigationGeneralBackground.

[ayush0402]

@rt4914 @anandwana001 PTAL.

[anandwana001]

I will be able to take a look at this on weekend.

[anandwana001]

my codeowners files, LGTM

[oppiabot]

Unassigning @anandwana001 since they have already approved the PR.

[BenHenning]

color_defs.xml LGTM for snake_case changes & replacing colors.xml.

[rt4914]

LGTM, thanks.