Thanks to great CodeMirror editor and its built-in syntax highlighter we now have a nice editor widget for Sieve scripts. So, now you can edit your scripts not only using the user-friendly interface, but also by directly changing the script code. Which is especially useful if you need to use a Sieve feature not supported by the UI.
This is a power-user feature and is optional, so admins can disable it on their servers. Many thanks to Martin Fischer who implemented the initial version of the feature and provided pull requests. The feature will be available in Roundcube 1.3.