In the message panel you can configure the settings to be applied to the message.
Content Area Width: sets the width of the content area. This is used when the device has a screen larger than that width. You can consider it as a maximum width for that content area. On small devices, it will be ignored.
Background color: the default background color for the message. You can override this at the row, column and content block level when you configure the properties of those elements.
Content area color: the default background colour for the content area, which is the area for which the content area width is set above.
Default font: the default font to be adopted by all elements of the message.
Link color: the default colour that all links in your message will have, unless you overwrite them as individual items.