Hello, @Jon!
If you registered your theme, you should be able to update your theme through the WordPress backend.
Anyway, if you already successfully updated the theme to v. 1.4.9.5. and have a problem with the customizer, then you most likely have the incompatibility with some of the currently active plugins.
If you want to discover what exact plugin causes the error, you can temporarily disable all the active plugins, then if the problem is fixed, activate the plugins one by one until the problem reoccurs, so on that way, you will find the problematic plugin(s), then replace it with some similar or let us know what exact plugin causes the error, so we can try to fix the incompatibility.
Regards