GKrellM is a single process stack of system monitors which supports applying themes to match its appearance to your window manager, GTK, or any other theme. It could be seen a GUI equivalent to Conky.
It is easy to configure and runs well on Linux, macOS, and Windows. It will show monitors for a host of sensors and activities on your PC, and you can even set alarms to alert you when thresholds are exceeded. There are also numerous plugins that further extend its capabilities.
My video shows what it looks like, what sorts of themes there are, what menu options there are, and how to set audible alerts. I also contrast various aspects of GKrellM with Conky itself.