fix hex pattern

[AdrienC]

Strings starting with # and followed by 1, 2, 4 or 5 chars are not valid hex colors.

[phax]

I understand your request, but I am bit reluctant to add this, because it is a breaking change.
When reading existing CSS, it should be identified as colors, but when creating new CSS of course it shouldn't be possible to create new, invalid values.
But since the RegExg is basically only used in ICSSProperty.isValidValue which can be disabled, there is a possible to workaround in case of issues.
So lets try it and see if there are compalints ;-)