Prevent Mac Terminal from reverting to default theme
Solution 1:
You need to select which profile you want to be your default in the preferences for the Terminal application. Here is a way you do that:
Open the Terminal.app
Select "Terminal" Menu then "Preferences..."
Select the "Profiles" tab (labelled "Settings" before Yosemite)
Click and highlight a theme listed under Profiles.
Press the "Default" button near the bottom of the window.
Open a new Terminal window, it should now have the theme that you selected as the new default.
Note: Existing terminal windows will not change to the newly selected default theme, you will need to close and re-open terminal windows to get the theme to be applied.
Solution 2:
Terminal -> Preferences -> Settings -> Click desired Profile -> Click "Default" at the bottom of the frame.
I would also suggest iTerm2 over Terminal.app
Solution 3:
Per the Preferences image in the Question - to set the default view, that's all correct.
BUT - to set the initial / startup view i.e; the first Terminal window that comes up, you have to set that in the Startup settings (first tab in preferences window).
What you put in the Startup tab will show when you first launch Terminal.
What you set as the Default theme will be any additional windows that you open after.
Solution 4:
The "Terminal" -> "Preferences" menu opens up the settings. You can add or modify different types of windows, and, choose your favorite one as the default with the button at the bottom left of the settings window.