Correct me if I’m wrong, but from my understanding such integrations are not a part xwiki-platorm. This image Lightbox application is in the standard distribution, so in my opinion it should be in line with the style of the standard distribution.
The white dropdown change is not okay, I’ll update my proposal for solving this ticket while keeping the hardcoding 
<EDIT> The proposal is updated. </EDIT>
Leaving this topic open to finish the discussion 