GUI tray uses inadequate color scheme

### ⚠️ 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

The GUI uses colours for some elements that make texts illegible under Plasma with KUbuntu light and Breeze classic colour schemes). Text and glyphs are light where they should be dark.

### Steps to reproduce

1. Open Nextcloud Desktop.

### Expected behavior

All elements should be legible.

### Which files are affected by this bug

n/a

### Operating system

Linux

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

KUbuntu 26.04

### Installation method

Community PPA (please test with the AppImage package ?)

### Nextcloud Server version

various

### Nextcloud Desktop Client version

33.0.5daily

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

Clean desktop client installation

### 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

_No response_