It is painful to change icons in applications like Gimp or Libre Office (LO), especially if you are working in many systems. Having different icon sets from one system to another it won’t work. However if you are running LO into a single system it will be okay I guess.
LO in version 4.2 added a new flat monochromatic icon set called Sifr, but not everyone knows how to switch on this.
That’s LO 4.3, and icons may be slightly different in 4.2
To change Icon Set navigate to:
Tools -> Options -> View # Icon Size & Style