Fix katex font

[st3iny]

Fix #112
Fix #168

The font of katex spans gets overwritten by a rule in preview.css. I fixed it by applying said rule to the preview container only. Font settings will be inherited to all children anyways.

Before

After

[ffranchina]

Thanks for this fix! Is there any news about its merge?

[ffranchina]

This fix solves a problem that, by default, appears using mermaid, too.
The code

```mermaid
  graph TD;
    a-->0123456789;
```

produces

Basically mermaid expects to use its own font for computing the dimensions of the boxes. Without the fix, the CSS override every font usage from third-party plugins.

[icewind1991]

Thanks!

[LeducH]

How can I use this workaround. I am on version 2.3.6 from the store. Is there a way I can integrate that change?

[LeducH]

I tried to change the file in the folder nextcloud_data/custom_apps/files_markdown/css/preview.css with the changes without any change is there a way around it?

That will work as a workaround I just had problem with the cache in my browser