Quoting Andreas Nilsson <nisses.mail@...563...>:
Apparently my build (13378 ...the latest one I got to work) wants to use 16x16 images for the icons in both the topmost (although the ones it gets from the desktop theme is 24x24) and the secondary toolbar. The tools toolbar however still use 24x24 images (and that is the correct behavior I suspect).
I looked into things a bit, including your screenshots. Those were quite helpful.
First, the two horizontal toolbars are both the "secondary" size, while the vertical is "primary" size.
Now... there is a bit of a bug at times where we might fetch the size a little too early, before the theme has changed it. Then there is also the case where a theme reports a different size from the icons actually used.
One of those two might be hitting you. If you're using Inkscape's "icons.svg" approach to place them, they *should* be consistent with the icons provided by the stock mechanisms. If you're installing the icons separately as the stock ones, then having enough sizes and listing them correctly with GTK+ should be the key.