When you choose to Render Thumbnails from Geometry, you can also choose to style the rendered shapes themselves. To apply Styles to the rendered shapes:
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In the Thumbnails > Render Images from Geometry pane, check Style Rendered Shapes
Choose Point, Line or Polygon (you can then further select MultiPoint, MultiLine or MultiPolygon from the Shape Type drop-down menu)
Adjust the settings in the Style box
stroke-color and fill-color use standard hex values as input
stroke-width sets the width of the line or polygon - enter 0 for no line
fill-opacity and stroke-opacity control how transparent the shape is on a scale of 0 to 1, where 0 is transparent and 1 is completely opaque
For Points, you can adjust shape (circle or square), fill-color andfill-opacity
For Lines, you can adjust stroke-color, stroke-width and stroke-opacity
For Polygons, adjust stroke-color, stroke-width, fill-color and fill-opacity
Click Preview to see those settings in the Preview box
Click Save when you are done