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The background colour of the toolbars is the same as the background colour of the overall window, which means the toolbars are visually "lost" and individual buttons are unclear.

Either
-- Add a single pixel contrasting border around the toolbars.
-- Alter the toolbar background to be a slightly different colour to the window background (darker in the light theme, lighter in the dark theme).
-- Both add a border and introduce a colour difference.

Any visual adjustment should also ensure that individual buttons are bordered or coloured to be distinct from the adjacent buttons.

Sync the changes with those of suggestion #13921

Does 9.x not have per button the context menu option "Begin a Group" anymore? In doubt you could set it for every button of every toolbar you have. This screenshot is from 8.4.2 where I need almost no toolbar and just added 4 most used actions right into the menu bar to not even waste space for a new line of toolbars:

Any visual adjustment should also ensure that individual buttons are bordered or coloured to be distinct from the adjacent buttons.

Version 9.5 offers a range of new options to change the theme.
The later versions of VS and Office all have this problem, but most prior to 2010 seem viable.
VS 2008 seems best.