I have used a custom version for a while, which add multiple border styles and fixes a couple of bugs.
To keep a version compatible with TB60 online, I have now released it as "BorderColors D".
An essential tool for those using many email identities... but not too many.
A few months have passed but the issue described by Drachsi below is still a problem: the configuration dialog does not expand if you have more than a few indentities.
If that's a deal-breaker for you, you can try my experimental version fixing the issue and adding multiple frame styles.