I have Libreoffice 6.1.6.3 on my Ubuntu (18.04.2 LTS) at home and my Windows 7 at work.
When I open Writer, Calc, etc. on the Ubuntu machine the tools bar icons are different looking then the those on my Windows 7 machine.
Is there an explanation. I prefer Ubuntu icons (more traditional) over the Windows 7 icons which I call "stick figures:.
Is there an explanation
Yes there is 
LibreOffice includes several icon sets (those that live in the project, and developed as part of it): Breeze, Colibre, Elementary, Karasa Jaga, Sifr, Tango. Any of them is available to be chosen on Options▸LibreOffice▸View▸Icon style (on Linux, distros may split the icon sets to different packages, and allow to install them separately, so the installed set may not include all of them).
Each of the icon sets follows some well-defined style; and some of them follow HIGs of some well-known environments. Specifically Colibre follows current HIG for Windows applications by Microsoft; Breeze follows KDE recommendations; for Gnome, Elementary fits well. And LibreOffice includes code that checks if user hasn’t yet manually selected an icon set, and in that case, automatically selects the set that fits current environment, to provide most native look and feel. But as usual, user’s choice has the highest importance, so when a theme is selected explicitly (i.e., no “Automatic” is selected in the list), it will be used independently of current environment.
Possibly you should install additional icon themes from Ubuntu repo, if you’ll can’t find need in dialog as Mike answered