Changing a diagram's look is a matter of picking a new theme and watching the canvas update. livediagram brings the theme picker front and centre in a dedicated dialog, so there is plenty of room to compare looks.
Opening the Dialog
Click the paintbrush button in the bottom-right corner of the editor, beside the zoom controls (it is there on every screen size, phones included). It opens the Tab Look & Feel dialog on its Theme tab. You can also run Open theme from the command palette. The dialog also has a Canvas tab for the backdrop pattern and colours and a Font tab for the tab's default font, and you can switch between them from the tab strip below the header.
The paintbrush only appears for people who can edit the tab, so it is missing on a view-only link, in zen mode, and in Draw mode.
The dialog is an editor, not a wizard: there is no Apply or Cancel. Every change applies live to the active tab, and closing it (with the X, the Escape key, or a click outside it) simply dismisses it without rolling anything back. Undo takes a theme change back like any other edit.
Browsing by Category
The Theme tab opens on an overview: a Default quick-pick plus a card for each category (Formal, Cool, Warm, Dark and Multi-colour), alongside a Custom category for your own saved themes. If the tab already uses a theme from a category, the dialog opens straight into that category.
Click a category to drill into its themes. All themes at the top steps back out.
Click a theme to apply it live. Double-click a theme to apply it and close the dialog in one move.
The themes inside a category are shuffled each time you open the dialog, so different ones greet you first. While you browse the Dark themes in light chrome (or a light category in dark chrome), a Turn on Dark mode (or Turn on Light mode) switch below the grid matches the editor's own appearance to what you are looking at.
To build on a built-in theme, hover its card and press Copy: the theme builder opens with its colours filled in, ready to save as a custom theme.
Recolouring Elements
Applying a theme sets the tab's backdrop and recolours the elements on it, but keeps colours you have hand-set on individual shapes and text, so deliberate customisations survive the switch. Arrows are the one exception: every arrow's stroke always snaps to the new theme's colour, because connectors left on an old hue read as broken. Sticky notes keep their own note colours.
If you want a clean slate, Reset elements to theme at the bottom of the Theme tab repaints every element on the tab to the theme's defaults, overriding your customisations. A shape carrying a style preset keeps its preset, redrawn in the theme's colours.
Use double-click to apply and close once you have found the theme you want, and single clicks while you are still comparing options.
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