[Broken Website] Proton Inbox not properly themed (content area of emails)

### Prerequisites

- [x] I [searched for any existing report](https://github.com/darkreader/darkreader/issues?q=is%3Aissue) about this website issue to avoid opening a duplicate.
- [x] I can reproduce this website issue in a new, unmodified web browser profile with Dark Reader installed as the only extension.

### Website Issue Description

Hello,

just a quick report about an issues while I'm trying to switch from GMail to Proton. Proton already has a very good theming and normally does not need Dark Reader at all. However, theming does not include the content area of emails as with all other of such cases. BUT UNFORTUNATELY, I cannot fix this by using Dark Reader like I did with GMail, because it ignores this area too!

Any hope to fix this? Having to stare on bright white area inside a website which is dark all around it is even more annoying as a completely white one and since Dark Reader, as the usual solution, falls short here ... =(

Screenshot of my GMail inbox with Dark Reader enabled:

Screenshot of my Proton inbox with Dark Reader enabled (same email):

Please help!

I'm using Dark Reader with its default styling settings.

### Website Address

https://mail.proton.me/

### Steps To Reproduce

Open Proton inbox usind URL above.

### Screenshots

Added above.

### Operating System

Windows 11 Pro 25H2 (doesn't matter since happening on others, too)

### Web Browser name and version

Vivaldi 7.6.3797.63 (doesn't matter since happening on others, too)

### Dark Reader version

4.9.113

### Additional Context

N/A