Pink lines are showing after exporting from Inkscape

[Pinknoise]

Hello! I have very little experience in theming I'm reading Walter guide and trying little modifications.

I imported volumethumb.png in Inkscape, draw a quick red button over it to test, export and put it back in the Theme folder. In this case Solaris LCS Blue V2. Thank you Lucas for this nice theme !

I created a new TCP called TCPBus I simply want the volume knob red to differentiate busses but here how it is:




So what makes the pink lines on the original png disappear while the ones from my exported Inkscape png show up ?