You can use a theme to personalise the 'look and feel' of Confluence. Some themes simply change the basic styling, others add new functionality or significantly alter the appearance of Confluence. You can apply a theme to your entire Confluence site and to individual spaces.
Confluence comes with a selection of themes.In addition, a site administrator can install new themes as plugins via the Confluence Administration Console .Provided that the theme is available in your Confluence site, any space administrator can apply a theme to a space. By default when you create a new space, the space will have the Confluence default theme.
To apply a theme across the site:
Choose the cog icon, then choose General Configuration under Confluence Administration
Choose Themes in the left-hand panel.
The screen will display all available themes. Select a radio button to choose a theme.