Themes in Publii define the overall look of your site; they decide the layout of posts and pages, and how the frontpage looks. Publii comes with many themes preinstalled for you to choose from; it's a good idea to select a theme as early as possible to make sure that your site looks the way that you want. In the event that no theme is chosen, Publii will set the standard Simple theme by default.
To choose a theme, open the Publii Settings by clicking on the Settings option in the left-sidebar. There are several options here, but for now we are only interested in the Theme option. Click the drop-down list next to the Theme option (it is the third one down from the top). This will display a list of available themes; simply select the one you wish to use, and then click the Save Settings button. Your theme is now ready and your posts/pages will be styled appropriately. You may notice that the drop-down list of themes includes several sections; Use, Install and Use, and Uninstall. This is because the themes are included with the Publii download, but are not copied to your site's files until you specifically decide to use them, to reduce the size of your website. Initially, all the available themes will appear in the Install and Use section; once a theme has been selected, it will be included in your site files and will now appear in the Use and Uninstall sections too.
If you are just setting up your site for the first time, the next step after choosing your theme is to configure Publii so that it can connect to your server and upload or update your website whenever needed; check out the Server Configuration section in the left sidebar for more details.
Translating a Theme
Publii includes support for language .json files and overrides that can be used to translate non-content areas of a theme, such as button labels. The main language file for each theme can be found in the respective theme folder wherever your Publii docs are stored, such as publii\themes\themeName\themeName.language.json. To translate the theme, an override version of the language file with the same name must be created in the sites\siteName\input\languages folder (the languages folder may need to be manually created first). This file will then be used as an override for the base language file.
For a full step-by-step guide to theme translation, and a breakdown of how it works, visit the Publii translation guide article.