Dark mode: poor text contrast, incorrect theme and avatar display in account menu

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I have searched for it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

### "Delete the account" button text is difficult to read

When opening the client's graphical interface, we click on the menu with our username. At the bottom of this menu, there is a button to delete the account. Unlike in light mode, the button text is difficult to read in dark mode due to poor contrast.
> See Image 1 where the unreadable button text is circled in red.
> _This is similar to these bugs: #10431, #6070, #6554, #10604_

---

### Graphical interface does not follow system theme (dark/light)

In the same interface as before, when switching the system's theme from light to dark or vice versa, the graphical interface remains unchanged
> See Image 1, where the system is set to dark mode while the graphical interface remains in light mode and Image 2 for comparison.
> _This is similar to these bugs: #7792, #10604_

---

### Default avatar is briefly displayed when switching themes

When switching between light and dark mode, the user's avatar is temporarily replaced by the default avatar.
After a few seconds, the user's actual avatar is displayed again.
> See Image 1 where the user's actual avatar is displayed normally, and Image 2, where the user's avatar is replaced by the default avatar after switching the theme.

---

Image 1: Graphical interface in dark mode

Image 2: "Graphical interface in light mode

### Steps to reproduce

1. Launch the Nextcloud desktop client.
2. Click on the Nextcloud widget icon and select "Settings".
3. Switch the system theme between light and dark mode.

### Expected behavior

The Nextcloud graphical interface should correctly follow the system theme when switching between light and dark mode.

The user's avatar should remain visible without being temporarily replaced by the default avatar.

The text including "Delete the account" button, should be clearly readable in dark mode.

### Which files are affected by this bug

None

### Operating system

Linux

### Which version of the operating system you are running.

Debian 13.6

### Installation method

Official Linux AppImage

### Nextcloud Server version

34.0.0

### Nextcloud Desktop Client version

34.0.2

### Did this occur after an update or on a clean installation?

Major version update (i.e. 4.0.0 → 33.0.0)

### Are you using the Nextcloud Server Encryption module?

No

### Are you using an external user-backend?

- [ ] Default internal user-backend
- [ ] LDAP or Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell

```

### Additional info

I've already noticed the bug with de "Delete the account" button text" difficult to read in the previous version (34.0.1). It's not a new bug that appeared in the new release of Nextcloud.

Reproduce the three behaviors in the desktop client's account menu by switching the Linux system theme, then trace the account menu, theme handling, and avatar display entry points. No affected files are identified, so confirm the relevant area before changing anything; done means the interface follows both themes, dark-mode text is readable, and the user's avatar does not briefly disappear.

Written by the indexing model from the issue text.