Where to find the Redmine theme?

There are several places to find Redmine themes. The official Redmine theme list on the Redmine wiki contains many free themes sorted by the year they were last updated, with the most recent ones first. Some popular themes listed include Gitmike, minimalflat2, and RedmineCRM themes. 

The RedmineUP website has a guide on how to install Redmine themes. It explains the steps to copy and unarchive a theme in the Redmine root folder, then activate it in the Administration settings.

The Redmine wiki also has a guide on how to create a custom Redmine theme. It explains how to create a theme directory in public/themes, add a custom application.css stylesheet, and optionally add custom JavaScript and a favicon. The theme can then be activated in the Administration settings.

Not to forget the Redmine forums discuss where to find specific themes, like this one asking about the "Circle" theme.