fixes #7666, #7667, #7668 - errata mail notifications

You could create variables with the common css attributes and use those variables when styling.
See: variable (https://github.com/theforeman/foreman/pull/1910/files#diff-4b17df40367067144fb027cc5533ecefR1)
usage: https://github.com/theforeman/foreman/pull/1910/files#diff-4b17df40367067144fb027cc5533ecefR7