Dark theme cannot be configured as default for the Element instance (i.e. in Welcome pages)

### Steps to reproduce

1. Where are you starting? What can you see?
3. What do you click?
4. More steps…

Server OS: Ubuntu 20.04
Web server: Nginx 1.18
Desktop operating system: Kubuntu 22.04
Web browser: Firefox (latest current version)

I installed Element 1.11.23 on my server and set the default theme to Dark in my config.json (please see attached screenshot) below.


I browsed to to my Element instance at https://matrix.t25b.com.

### Outcome

#### What did you expect?

I expected to see the Element welcome page dark-themed.

#### What happened instead?

Instead, the Element welcome page was light-themed.


Blinded and dazzled, I fell off my chair.

This is actually a long-standing bug.

For me, the 'default_theme' setting in config.json is meant to apply to the welcome pages of Element, not just the theming of Element once I've logged in.

I want it dark, please. Preferably covered in hot melted chocolate, but at least DARK!

I want darkness from the moment I hit the login / registration / password reset pages.

Please could we finally fix this in the next release?

And thank you so much for your work on Element. 1.11.23 is another great release.

### Operating system

Ubuntu 20.04 Server Edition

### Browser information

Firefox on Kubuntu 22.04

### URL for webapp

https://matrix.t25b.com

### Application version

Element 1.11.23

### Homeserver

matrix.org

### Will you send logs?

Yes