Because CSS Filters and CSS Regions are not yet supported in all the browsers, Edge Reflow doesn’t have support for them turned on, by default. To use CSS Filters or CSS Regions in the tool, you need to go to the menu item: View -> Shiny Web Features. You will get the following dialog describing these features.
Turn these features on.
Now, you will be able to use CSS filters at the bottom of your Styling tab for any element on your canvas.
Or, you will be able to use CSS Regions which I have a tutorial for in an earlier blog post.