The file path that @chadc01e provided no longer exists on the current version, so I thought I'd update the topic for anyone interested in making this change.
As of version 2.4.4 p3, the bar graphs for the System Information widget on the dashboard (memory usage, temperature, disk & swap usage, etc.) are red regardless of the value.
Go to System > General Setup and in the webConfigurator section it will show your theme, our system uses pfSense.
Go to Diagnostics > Edit File and in the /usr/local/www/css/ folder open your theme (pfSense.css). Use the browser search feature (Ctrl +F) for ".progress-bar" and edit this section:
.progress-bar {
background-color: #B71C1C;
}
I changed the color from #B71C1C (red) to #4CAF50 (green) that matches the Thermal Sensors, Interfaces, and other dashboard widgets. When finished, click "Save" and reload your dashboard.
sys-info.JPG