If you need to start mythfrontend (or another Myth program) and temporarily override a setting in the database, simply use the -O command-line argument:
mythfrontend -O $somesetting=value
Note that command-line overrides are meant for short-term overrides. You should not normally need to use overrides. Instead, if you always need a specific value, set that value appropriately in mythtv-setup or mythfrontend settings.
For example, to start mythfrontend with the qt painter rather than OpenGL, use mythfrontend -O ThemePainter=qt
This is (still, in April 2019, with MythTV version 30) a very useful tool if you find that you've changed a setting to a value that stops mythfrontend from running. For example, if you set ThemePainter to OpenGL on a system that doesn't support OpenGL.
To start mythfrontend with the MythCenter theme, use mythfrontend -O Theme=MythCenter
Also available (as described by mythfrontend --help) is a geometry setting to allow you to resize and position the mythtv GUI: mythfrontend --geometry 1280x720
To start mythfrontend at 1920x1080 with the Minimalist-wide theme, use mythfrontend --geometry 1920x1080 -O Theme=Minimalist-wide