Convert css to scss.

[robinlinden]

It has a better syntax, more features, and is generally a lot nicer to work in than plain CSS.

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This change is 

[iphydf]

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Reviewed 6 of 6 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[cebe]

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Reviewed 6 of 6 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[cebe]

Reviewed 1 of 6 files at r1, 3 of 3 files at r2. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[iphydf]

Reviewed 1 of 6 files at r1, 3 of 3 files at r2. Review status: 2 of 4 files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[iphydf]

Reviewed 2 of 2 files at r3. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[cebe]

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Reviewed 2 of 2 files at r3. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable