The topic shape is by default set from the theme, or inherited from the parent, but you can override the shape by setting the shape using the shape inspector.
To change the shape of the selected topics, click on the shape that you want to use. If the shape is either the rounded rectangle or chamfered rectangle, the corner amount slider will be activated and you can adjust the shape.
If you use the cloud setting, the cloud shape slider will become active, allowing you to choose the shape of cloud that looks best for your topic.
If you use the custom shape option, then the custom shape buttons become active. You can then click on any of the four quadrants of the topic and choose what corner type you would like; square, angled left, angled right, rounded or chamfered. If you are using the rounded or chamfered shape, then you can set the corner amount using the slider.
This combination of shape options gives you almost infinite possibilities to shape your topics the way you want them.
If you want the descendant topics to also use the same shape instead of the default shape from the theme, then you can turn on the Children Inherit These Shape Settings option.
To revert to the default topic shape from the theme (or inherited from the parent), click on the Default Shape button.