Select the feature for which you want to change the styling. Note: you can multi-select by holding down the shift key then clicking multiple features.

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The feature details menu will appear on the right of the screen.

Click the styling button at the top right corner of the feature details menu. The styling options for that feature will appear. Note: styling options are different between polygons, lines and points.

It is then possible to alter the fill of a shape, and the fill's opacity.

It is also possible to alter the pattern of the shape and it's opacity, and the colour of that pattern:

It is also possible to alter the border colour and width:

Finally, by clicking 'Reset style', it is possible to revert to the original styling of that feature when it was created.