Make Qt-based `mozc_renderer` look more similar to Win32 `mozc_renderer`

## Description

As a follow up to the following issue:

* https://github.com/google/mozc/issues/1548

There is still some room for improvement to make it look closer to Win32 `mozc_renderer`.

I've locally played with it and confirmed that we can improve the UI as follows (as of Ubuntu 26.04).

### Suggest Window

| Current | Proposed |
| ------- | ---------- |
| | |

### Candidate Window

| Current | Proposed |
| ------- | ---------- |
| | |

This is a placeholder issue to keep track of the effort to upstream changes to achieve the above UI changes.

This is also a preparation before we start working on features like dark theme support.

## Steps to reproduce
1. `bazelisk build package --config oss_linux --config release_build`
2. `unzip bazel-bin/unix/mozc.zip -d ~/mozc-bin`
3. `sudo rsync -a ~/mozc-bin/usr/ /usr/`
4. `ibus restart`
5. Add Mozc an IBus input source. Then select Mozc as the current input source.
6. `gnome-text-editor`
7. Select Mozc. Type something.
8. Observe the suggest / candidate window

## Version or commit-id
0651fbcae495ff98f6ae17bb59e2bee0e3a79661

## Environment
- OS: Ubuntu 26.06
- IMF: IBus 1.5.34-rc2
- Qt: Qt 6.10.2
- Related Applications: Any