Enable Riverlea extension (*not* theme) + dark mode on new Standalone installs

[ufundo]

Overview

Enables Riverlea extension in the Standalone install process + set the starting dark mode to Inherit from browser/OS.

Note this does not select a Riverlea theme by default - but they will be available in the theme options in display settings.

Before

• default for Riverlea dark mode has been switched to Always light to avoid conflicts with CMS themes
• Two steps required to use Riverlea ( enable the extension, then select a theme )
• Standalone will be Always light by default

After

• Only one step to use Riverlea on Standalone ( go to Display Settings and pick one)
• Standalone will Inherit [dark mode] from browser/os by default

Technical Details

The slight risk of this would be if any of the Riverlea extension hooks caused problems when the extension was installed but a Riverlea theme wasn't selected. There were a couple of issues like this early on, but I think these have been ironed out now.

[civibot]

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[ufundo]

Thanks @colemanw

I think we need a few more Star Wars Dark Side references in the Standalone/Riverlea marketing material!