You can now quickly configure node border styling for the whole mind map using the MindMup theme editor. Here is how to do that:
From the application View menu, select the Customise Theme option.
A sidebar will show the theme customisation options. Scroll down to the Borders section (or click the headers of the currently open sections to close them). In the Borders section, click on the color selectors to update border colors, or the Thicker/Thinner line buttons to control the border thickness.
You can modify the styling for all the nodes in the “Defaults” section, or update styling for a specific level in individual level boxes below the Defaults box.
For more information, check out the How to change node border thickness or color tutorial.