Use the Diagram Options dialog to configure any diagram-specific settings. For now, this tab lets you mainly choose the font used to render all diagram elements.
By default, UMLBoard uses the Indie Flower font, but choosing any custom font installed on your system is very well possible.
The preview on the right side of the dialog will show you what diagram elements will look like when rendered with your newly selected font.
When using a custom font, you also can choose additional font parameters like the font weight and style, assuming that the relevant font types are installed.
When exporting your diagram to an SVG image, your custom fonts won't be embedded into the resulting SVG file. While your SVG diagram might look fine on your machine, it might look different on another device where your custom font is unavailable.
To ensure that your diagram gets rendered as expected, best export it in PNG format (e.g., if you want to embed it in your website). This problem does not arise when using the default font, as UMLBoard automatically embeds it into any exported SVG.
Starting with Version 1.7, you can also adjust the font size used for your diagrams.