[Annotations] "Color Picker" button is not highlighted when it's hovered

[Softvision-CristinaBadescu]

[Affected versions]:

• Nightly 60.0a1
• Screenshots 31.0.0 stage

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• The "xpinstall.signatures.required" boolean pref is set to "false".
• The "extensions.legacy.enabled" boolean pref is set tot "true".
• Have the Screenshots 31.0.0 stage version installed from here, on a new clean profile.
• Have a saved shot link.

[Steps to reproduce]:

1. Open the browser and navigate to the saved shot link.
2. Click the "Edit this image" button.
3. Hover over the "Color Picker" button and observe the behavior.

[Expected result]:

• The button gains a hover effect.

[Actual result]:

• The button does not have a hover effect.

[Notes]:

• Here is a screenshot with the issue:
  

[Softvision-CristinaBadescu]

This issue is no longer reproducible using Firefox Screenshots v31.3.0dev installed from here, on latest Nightly build 61.0a1 (2018-03-25), on Windows 10 x64, Mac 10.12.6 and Arch Linux 4.12.