For the longest time, I just thought adding cool border effects to a vector file was the domain of ArcGis Pro or that it was easiest to do there. I was wrong. Here are a couple of tips to add some flair to your borders in QGIS:

Add shaded offset border

1. In the layer styling panel (click on your vector, select the paint push thingy in the layers panel)pull-down, select inverted polygons in the symbol categorization menu
2. Click on Simple Fill below fill, and below that in the Symbol layer, type change to Shapeburst fill
3. In the gradient colors section, change both colors to black with the second black changed to full opacity, e.g., move the opacity slider all the way to the left so that color is transparent.
4. Under shading style, click Set distance and leave it at 5 mm, or if you want to minimize the shading, reduce it to 2 or 3.
5. Voila!

Inverted polygon to de-emphasize basemap

1. In the same vector border, open the layer styling panel and make sure that the Symbol layer type is Simple Fill.
2. Change the fill color to black (or whatever color you want).
3. Dial down the opacity to something 30-50% to highlight your area of interest and de-emphasize the basemap.
4. This effect, in fact, is easier to do in QGIS than ArcGIS Pro, so there's a win for Free and Open Source!

You're welcome!

V