Always use dark mode, on Mojave
Create a script to enable dark mode
This shell file will run an AppleScript command to enable Dark Mode. The first time this script is run, it may require permission to modify your system settings.
It is critical to mark the file executable with chmod +x - otherwise, it will not run and you will get a vague 'Load Failed: 5: Input/output error' from the launchctl service.
echo &quot;osascript -e 'tell application \&quot;System Events\&quot; to tell appearance preferences to set dark mode to true'&quot; &gt; ~/AlwaysDarkMode.sh
chmod +x ~/AlwaysDarkMode.sh

Create a launch agent
Create a new text file in ~/Library/LaunchAgents titled AlwaysDarkMode.plist (or something else, if you'd like). Paste in the following contents, ensuring to change the username to your own. You cannot use relative paths like ~/AlwaysDarkMode.sh - you must write out the full path to the file.
&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;!DOCTYPE plist PUBLIC &quot;-//Apple//DTD PLIST 1.0//EN&quot; &quot;http://www.apple.com/DTDs/PropertyList-1.0.dtd&quot;&gt;
&lt;plist version=&quot;1.0&quot;&gt;
&lt;dict&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;AlwaysDarkMode.sh&lt;/string&gt;
    &lt;key&gt;ProgramArguments&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;/Users/YourUsername/AlwaysDarkMode.sh&lt;/string&gt;
    &lt;/array&gt;
    &lt;key&gt;RunAtLoad&lt;/key&gt;
    &lt;true/&gt;
&lt;/dict&gt;
&lt;/plist&gt;

Test the launch agent
Because the plist is stored in ~/Library/LaunchAgents, it should run automatically when logging in. However, it might request permissions the first time it runs, so you can load it manually with launchctl load ~/Library/LaunchAgents/AlwaysDarkMode.plist.