By default, SmartGit 19.1 should follow the system light/dark theme (the Theme-option “Automatic select light/dark (recommended)” is selected). If, for whatever reason, you want to see a dark SmartGit on a light system (or visa versa) like you did in SmartGit 18.2, select the light or dark system independent theme option. Without any further changes, this will cause a couple of visual glitches because of following issues between SWT and the new used macOS API: bugs.eclipse.org/bugs/show_bug.cgi?id=549046 bugs.eclipse.org/bugs/show_bug.cgi?id=549174
You can mostly work around these problems by adding following line to ~/Library/Preferences/SmartGit/19.1/smartgit.properties:
org.eclipse.swt.display.useSystemTheme=false