"To choose a theme" means to decide on a particular subject or idea that will be the central focus or underlying concept of something, such as a party, event, presentation, or artistic creation. Full definition
More abstract ideas can work, too: seasons, colours or childhood memories all make excellent starting points for choosing a theme for the celebration. (sense-organics.com)
In this regard, the founding Editors carefully chose the theme of «access to justice» rather than «access to law». (uwindsor.ca)
As a theme addict, one of my criteria on choosing a theme is customization. (crackberry.com)