Creating a new theme
To create a new theme (or edit an existing theme), you need template editing user permission in LimeSurvey as well as permission to manipulate the files in the underlying operating system hosting your LimeSurvey installation.
Note: If you change themes, you could also affect all the pre-existing surveys from your LimeSurvey installation. Themes are an advanced feature that require experience and HTML knowledge.
The preferred way to create a new theme is through the Theme Editor (see below), which is located under Configuration on the top toolbar.
Some users prefer to work directly with theme files so that they can use their favorite text editor instead of the web interface. In this case, still use the Theme Editor to first create your new theme. This will create a new base theme with all the files you need in the LimeSurvey_web_root/upload/themes/survey/your_new_template directory. From there you can use your text editor to manually adjust the template files.
Configuration -> Advanced -> Themes -> Theme editor
Permissions Note: on Unix/Linux systems, these theme files will be owned by the group and user where the web server is running (may be "www" for some systems). So, make sure you have the proper permissions to edit these files, When you save them, make sure they don't change ownership! This way, you can still use the web Theme Editing interface if needed.
Themes files are a complex feature. Besides HTML, you should also be familiar with CSS, Twig, and JavaScript.
Please refer to the other articles in the themes section or more details.