The theme uses a very slight 'off' (coloured) grey #8e8d8c, source:
https://wiki.documentfoundation.org/Design/Whiteboards/Flat_icon_set#Guidelines
Is it worth exploring using a pure greyscale, like #8d8d8d ? source:
http://www.colorhexa.com/8e8d8c
I'm not sure the colouration is visible or detectable in use, but it hurts compression of the resulting .png icon files (which could be shrunk if the images contains just 256 shades of grey+ alpha).
The theme uses a very slight 'off' (coloured) grey #8e8d8c, source:
https://wiki.documentfoundation.org/Design/Whiteboards/Flat_icon_set#Guidelines
Is it worth exploring using a pure greyscale, like #8d8d8d ? source:
http://www.colorhexa.com/8e8d8c
I'm not sure the colouration is visible or detectable in use, but it hurts compression of the resulting .png icon files (which could be shrunk if the images contains just 256 shades of grey+ alpha).