The advantage of CSS is we don't need to drop Adwaita to do our own
thing... we can override part of the Adwaita theme. I plan on making
the +/- buttons narrow by default.

• Modifying part of the CSS quickly leads to inconsistencies in the UI. For example if we change the spinbutton in the new slider widget it will look dissimilar to normal spinbuttons, so we should probably change those, too, and with them probably other buttons/entries and sooner rather than later we end up with *a lot* of custom CSS that will be hard to maintain, especially with the second point in mind:
  
• How can we ensure compatibility with system/custom themes? If we restyle our widgets we easily override CSS from other themes. For example the spinbutton CSS from the test program already broke the win32 theme I was experimenting with and I doubt other themes are more robust...

The only solution I could come up with (and it's not satisfying): Pick one theme (that's about as much as we can maintain) and force Inkscape to use it. Are there any alternatives I overlooked?

I've done a search through the GUI and have found that the
GimpSpinButton occurs in the following places:

* Objects dialog:

  Included via the SimpleFilterModifier widget.

  Note: Opacity is handled by a hand-rolled Scale and SpinButton
combo. 

  The SimpleFilterModifier widget should be replaced by the
CompositeSetting widget which includes the SimpleFilterModifier widget
and a SpinScale widget for opacity.

* Layers dialog:

  Included via the CompositeSettings widget.

  Note: There are two instances of the CompositeSettings widget here
which is strange as the CompositeSettings widget can handle both.

* Fill-Stroke dialog:

  Included via the CompositeSettings widget.

* Filter Effects:

  Included in various primitive sections via the SpinScale and
DualSpinScale widgets. The labels inside the GimpSpinScale widgets are
left blank and instead are put in a separate label widget in front of
the SpinScale and DualSpinScale widgets. This could be changed to save
space.

BTW, There is a spin-slider widget that is still in our code base but
hasn't been used since 0.48. It is what the filter effects used then.

I don't see any particular case where we are seriously space
constrained but if necessary we can replace the SpinScale by a simple
SpinButton.

Personally I don't see an easy option with the current designs. I 
actually tried to stack the +/- buttons again which could be
realized 
with CSS (see the win32 theme for an example - you can select it
from 
the "Visual" tab of the gtk inspector) but even then we still do not 
really gain enough space (and the gtk-devs were not wrong: the
buttons 
become tiny).
So maybe it's time to re-think spin slider widgets in toolboxes from
the 
start and I'd say we should brainstorm this before finalizing on any 
design, otherwise we might just need to start over again once we
realize 
the new widget did not gain us anything except a GPLv2+ licensing...

I would be happy to entertain a better solution but in the meantime
I'll go ahead with replacing the GimpSpinScale widget now. While it may
not be an optimal solution, it is better than what is there now.