[New study screen] the screen flashes noticably upon opening the reviewer in the dark theme

### Checked for duplicates?

- [x] This issue is not a duplicate

### Does it also happen in the desktop version?

- [x] This bug does not occur in the latest version of Anki Desktop

### What are the steps to reproduce this bug?

Open the new study screen in the dark theme.

https://github.com/user-attachments/assets/f54fcf58-c543-4024-9340-8b0a0208fc7b

[It's somewhat random, so you may have to try a few times. If you quickly open and close the reviewer, it loads instantly, so wait a bit before opening it again.]

### Expected behaviour

The "loading background color" should at least match the theme (I think `#303030`).

Ideally, the loading would be faster. It feels a tiny bit slower than the old reviewer.

### Debug info

```text
AnkiDroid Version = 2.23.0beta3 (5457f48e80c00ee28ae5edc91569facb6b7f75bf)
Backend Version = 0.1.62-anki25.09.2 (25.09.2 3890e12c9e48c028c3f12aa58cb64bd9f8895e30)
Android Version = 13 (SDK 33)
ProductFlavor = full
Device Info = Xiaomi | POCO | mondrian | mondrian_global | 23013PC75G | qcom
Webview User Agent = Mozilla/5.0 (Linux; Android 13; 23013PC75G Build/TKQ1.220905.001; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/142.0.7444.106 Mobile Safari/537.36
ACRA UUID = b54212d4-c40d-479f-85b6-5654a05c2c11
FSRS = 5.1.0 (Enabled: true)
Crash Reports Enabled = true
```

### (Optional) Anything else you want to share?

If the timer is enabled, the review counts suddenly shift upwards when the timer pops up:

https://github.com/user-attachments/assets/bc116cbe-5a85-4535-b606-73b458cfaf01

### Research

- [x] I have checked the [manual](https://ankidroid.org/docs/manual.html) and the [FAQ](https://github.com/ankidroid/Anki-Android/wiki/FAQ) and could not find a solution to my issue
- [x] (Optional) I have confirmed the issue is not resolved in the latest alpha release ([instructions](https://docs.ankidroid.org/manual.html#betaTesting))