It should be upgraded to be consistent with the VDG guidelines. It's important, yes.

Well, Synaptic is a Gtk app and isn't part of the KDE software collection. At best we could make adjustments to the Breeze Gtk theme, but don't expect anything dramatic.

Perhaps you might consider trying out the Discover and Muon package managers which are built on Qt/Kde technologies and feature newer designs and better (visual) integration with the Plasma desktop.

As for a comparison between the 3, Synaptic is (and will always be) a very advanced, technical package manager, allowing someone who knows what they are doing to get things done quickly.
Muon is very similar to Synaptic but a bit more user-friendly. You might find that Muon will cover the vast majority of what you would do in Synaptic.
Discover is closer to some of the newer App Store style managers, focusing more on providing a great experience for users looking for (desktop) apps like Firefox, Krusader, Inkscape, etc. with screenshots, reviews, ratings, etc.

I don't know which distribution you are using but Octopi is very similar to Synapic but based on Qt and fits much better.
https://octopiproject.wordpress.com/

Well, Synaptic is a Gtk app

I know. Mildly put, I don't like the gtk toolkit, that's why I've crossed to plasma n Qt.

Hm, then you should be aware that nobody in this forum has anything to do with synaptics and your inquiry is not going to reach the right people here

Muon and Octopi look good to me.