Created attachment 201510 [details]
Writer window, Light, Standard UI (no separation)

I'm using LO 26.2 with GTK3.

There is no separation between the background area of the LO toolbar and the background of the main area of the window. This occurs both with Light and Dark themes, and is not module-specific (although in Calc, for example, we have the formula bar which serves as an effective separator, so the problem is quite minor). 

This does not happen with the Tabbed UI though - there's a background color difference.

It also doesn't happen with QT5 as the VCL (although I only tested in Light; Dark doesn't seem to work with QT5). I will say that with QT5, the tabbed bar has both a different background color and a separator line (or even a separation which looks like a groove; I like that).

Will add some screenshots to illustrate.