To create a new theme in LimeSurvey, users need the Themes permission along with file system access on the server hosting the installation.
Theme development is advanced work, requiring knowledge of HTML, CSS, Twig, and JavaScript. Keep in mind, too, that once a theme is in use, editing it can affect every pre-existing survey that relies on it.
Use the Theme Editor, accessible via Configuration → Advanced → Themes → Theme editor.
Extend an existing theme rather than starting from nothing. To create a new theme, select an existing core theme as the parent in the Theme Editor. Your new theme inherits the parent theme's templates, styles, and scripts, so you override only the files you want to customize while the rest stays maintained by the parent theme.
After creating your custom theme, open the survey, go to Settings → General → Theme, and select your new theme from the list. The survey then uses your custom theme and its inherited settings.
The following applies only to self-hosted installations. On LimeSurvey Cloud you create and manage themes through the Theme Editor and never need file system access.
Self-hosted, the Theme Editor generates the theme structure in the LimeSurvey_web_root/upload/themes/survey/your_new_theme directory, and developers can edit those files with an external text editor. On Unix/Linux systems, theme files must remain owned by the web server user — if an external editor changes ownership, the web interface can no longer edit them.