Hi Rogue and welcome!
The style (weight, colour etc.) of very single line can be changed in your LO documents from the Shape styles dialog (you will need to preselect what you want to change).
Also the styles of the SU drawing can be altered once you explode your model in vector or hybrid rendering mode but be aware that this will ultimately break the connection with the original SU model (practically it will make a 2D "snapshot" of what you can see in the SU model window).
As for your cylinder, I am not sure what's happening there although have a clue; in SU, go to your style dialog > in model > edit > edge settings and set profiles to 1 pixel (so that curved profiles will also have an outline but not thicker than a "normal" edge).
