Fix CodeSniffer violations for CSS files

Fix violations for
Squiz.CSS.ColonSpacing.After
Squiz.CSS.DisallowMultipleStyleDefinitions.Found
Squiz.CSS.EmptyClassDefinition.Found
Squiz.CSS.Indentation.Incorrect
Squiz.CSS.LowercaseStyleDefinition.FoundUpper
Squiz.CSS.SemicolonSpacing.NotAtEnd