Chicago95 can take that a step further if you like. I'm running KDE lately too (Slackware-current finally got KDE5 recently so I'm trying it out), but XFCE is the old trusty default for me.
Link to the project with Screenshots: https://github.com/grassmunk/Chicago95
For Qt-apps I use the "Windows" style so they fit together nicely.
Comparing Memphis and Chicago, there are some layout-bugs in Memphis, but I feel like it's overall a bit better balanced than Chicago.
I think all new themes are too bright and flat, Windows 9X was perfect (at least in that regard).
OpenLook https://b00merang.weebly.com/solaris-8.html
Motif https://b00merang.weebly.com/solaris-9.html
The Solaris 9 theme works great with FVWM.