As with every upgrade to a new version of WoltLab Suite there's the question of plugin and style compatibility when upgrading to WoltLab Suite 3.1, too. This article is an overview of what Elevenfour plans for its styles and plugins.
Just a few minutes ago the brandnew version of Flavius for WoltLab Suite 3.1 was made available for all users. Apart from adjustments for WoltLab Suite 3.1 there are also some bug fixes and detail improvements include with the new version. As always: You can read the changelog if you head over to the download page of the style. There's also a new color variant of Flavius available exclusively for WoltLab Suite 3.1: orange.
Since there had to be some changes made to the color palette it is sadly not possible to directly upgrade from a previous version of the style. If you're upgrading from an earlier version please backup your personal changes made to the style first. Then uninstall the style. After that you can install the new version from Elevenfour's package server right from your ACP.
Most (S)CSS declarations should be compatible with the new version. If you should need some help, feel free to open a thread in the support forums (user accound required). Regarding the templates WoltLab included some new functions which might require additional work on templates. Flavius itself comes with some custom templates compatible with WoltLab Suite 3.1.