Themes are a way to customize the look and feel of Salesforce by changing the colors, images, and logos that appear on Lightning Experience pages. However, there are some limitations and considerations when using themes, such as: only one theme can be active at a time, and a theme applies to the entire org; themes apply only to Lightning Experience and do not affect Salesforce Classic or the Salesforce mobile app; up to 300 custom themes can be created, modified, or cloned from the built-in themes; Chatter external users see the built-in Lightning theme only and cannot see custom themes. Reference: https://help.salesforce.com/s/articleView?id=sf.themes_overview.htm&type=5