Prevent Mac Terminal from reverting to default theme
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.