bug: System theme not updating correctly when switching back to light mode

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS Sequoia 15.3.2

### Expected behaviour

System theme should use light mode when my Mac is in light mode and dark mode when my Mac is in dark mode.

### Actual behaviour

System theme is not being honored. It works when I switch to dark mode from light mode, but not the other way around. The theme stays dark when I switch back to light mode, and I have to run `q restart` for it to update.

### Steps to reproduce

1. Run `q setting autocomplete.theme system`.
2. Switch to dark mode from light mode in macOS System Preferences.
3. Notice that Amazon Q theme is still in dark mode.

### Environment

```yaml
[q-details]
version = "1.7.1"
hash = "7d766a2d0580db62fae5a4bc2e6eb63946d4d08e"
date = "2025-03-14T22:08:03.470338Z (5d ago)"
variant = "full"

[system-info]
os = "macOS 15.3.2 (24D81)"
chip = "Apple M1 Pro"
total-cores = 10
memory = "16.00 GB"

[environment]
cwd = "/Users/USER"
cli-path = "/Users/USER"
os = "Mac"
shell-path = "/bin/zsh"
shell-version = "5.9"
terminal = "Ghostty"
install-method = "unknown"

[env-vars]
PATH = "/Users/USER/.console-ninja/.bin:/opt/homebrew/opt/postgresql@17/bin:/Users/USER/Library/Python/3.12/bin:/Users/USER/Library/pnpm:/Users/USER/.local/state/fnm_multishells/46777_1742459726570/bin:/Users/USER/.bun/bin:/Users/USER/.rbenv/shims:/Library/Frameworks/Python.framework/Versions/3.12/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Library/Apple/usr/bin:/usr/local/MacGPG2/bin:/Applications/Ghostty.app/Contents/MacOS:/Users/USER/.local/bin"
QTERM_SESSION_ID = "3694f6d78215453fa3fd3b90be03c75f"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "1.7.1"
SHELL = "/bin/zsh"
TERM = "xterm-ghostty"
__CFBundleIdentifier = "com.mitchellh.ghostty"
```