Icons for Clipman and Shutter are hard to see

[orschiro]

Tested applications:

• shutter 0.93.1-1ubuntu1
• xfce4-clipman 1.2.6-1build1

System: Linux Mint 18.3 Cinnamon

With Mint-X (pay attention to the two dark icons):

With Humanity just to test:

[JulianGro]

After looking into it, it seems that our problem is us using Mint X instead of Mint X Dark. The icons that look fine seem to just not be available in Mint X at all. Unfortunately Mint X Dark is unfinished #116