Theme Editor
To get to the theme editor you'll need to select the "Themes" option in the dropdown that appears when hovering your username in the site header. This is the first screen of the theme editor where you can add new, or remove existing, themes:
Clicking on a theme brings you into the editing mode. The deck embedded to the right is a preview of your theme and will update in real-time as you make changes. It has multiple slides in it for previewing and styling different types of content.
A save button appears in the top left when your theme has unsaved changes. In the top right you'll find additional options for your theme, such as CSS and HTML editors. These provide a greater level of control and customization.
Color Palette
You can define a custom color palette for your themes. This palette will be made available throughout the Slides editor, in any place that you can select a color.
Learn more about color palettes here: https://help.slides.com/knowledgebase/articles/419514-theme-editor-color-palette
CSS & HTML
The CSS editor supports LESS, meaning you can chose to write your styles as plain CSS or use nesting, variables and the other helpful features of LESS. All styles are automatically wrapped inside of a.reveal {}
selector when you save to ensure that styles don't spill outside of the deck.The HTML editor can be used to inject custom markup outside of the individual slides. This is especially helpful when adding fixed elements such as a company logo that always overlays the presentation.