Change default appearance of trusted S/MIME root certificates

We decided to remove the background color because it has very poor
contrast (with light color schemes where the text color is dark).
Moreover, in de-vs compliance mode compliant/non-compliant certificates
should use the positive/negative background color.

We decided to remove bold because bold should be reserved for the users'
own certificates.

• GnuPG-bug-id: T8275