Closes #236 (closed)
In my last comment on this issue, I wrote
Fixing this [...] would involve a proper "default style" mechanism in inkex
Good news: this now exists and it makes the fix almost trivial.
The MR also adds a unit test for randomizing gradient colors, which was previously not tested.
Note that the logic for handling opacity properties is now "if the opacity is unchanged or default after
modify_opacity, don't touch it". Therefore I had to update the unit tests.