Text is not highlighted in code blocks when using a custom theme

### Steps to reproduce

1. Add and select a custom Element theme, for example [Discord Dark](https://raw.githubusercontent.com/aaronraimist/element-themes/master/Discord/Discord-Dark/Discord-Dark-Theme.json) (instructions [here](https://github.com/aaronraimist/element-themes#how-to-use-themes))
2. Send a code block

3. Either
- restart Element (a custom theme will be used by default) or
- switch theme from Dark to a custom one _two_ times

### Outcome

#### What did you expect?

Code stays highlighted with different colors.

#### What happened instead?

Code is not highlighted. Animation:


### Operating system

Windows 10

### Application version

Element version 1.9.5

### How did you install the app?

From https://element.io/get-started

### Homeserver

matrix.org

### Will you send logs?

No

Reproduce the issue in Element using the linked Discord Dark theme, then inspect the custom-theme handling and code-block rendering paths. Verify behavior after restarting and after switching themes twice; done means code blocks retain syntax highlighting with different colors under the custom theme.

Written by the indexing model from the issue text.