stylelint: add color-hex-case rule
stylelint: add color-hex-case rule

Change-Id: I3c8308a54aa49d829110ea4758e8299bab60d6bc
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/310373
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite