[Bug]: Windows 10 Client UI styling issues

### ⚠️ 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've searched 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

I recently installed v3.15.0 of the Desktop Client in Windows 10.
This version fixed the previous darkmode/whitemode issues but a few new styling issues started to occur:
(Please let me know if I have to create separate bugreports for all styling issues)

1) There is a white angular border around the Client Window
2) The Button and general Styles look like they were suddenly from the year 2007
3) When you hover over the white icons, a white hover will display and you cannot see the icon anymore. (The darkmode has a better hover style)

### Steps to reproduce

Open Nextcloud Desktop Client 3.15.0 on Desktop

### Expected behavior

Should have a clear style like the versions before

### Which files are affected by this bug

styling

### Operating system

Windows

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

Windows 10

### Package

Official Windows MSI

### Nextcloud Server version

29.0.5

### Nextcloud Desktop Client version

3.15.0

### Is this bug present after an update or on a fresh install?

Updated to a major version (ex. 3.3.6 to 3.4.0)

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

Encryption is Disabled

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

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

### Nextcloud Server logs

```shell

```

### Additional info

_No response_

Start by reproducing the three reported styling problems in the Nextcloud Desktop Client 3.15.0 on Windows 10: the white angular window border, outdated button styling, and invisible icons on hover. No files or tests are identified; done means the listed UI issues have clear styling comparable to prior versions in both relevant display modes.

Written by the indexing model from the issue text.