Yes! There is a transparency slider as part of the colour picker:
NOTE:
Setting transparency to 0 will make the window non-clickable. This is a Microsoft Windows limitation unfortunately, you can set to 1 to get the most transparency possible without the window becoming un-clickable.
Is it possible to have the portals open on top of other objects? example: if I have a spreadsheet open and want to open another file, it would be ideal to be able to see the relevant portal where the file is without first having to minimize my first spreadsheet to access it