Make Qt-based mozc_renderer look more similar to Win32 mozc_renderer #1550

Description

As a follow up to the following issue:

• Candidate texts are rendered with white color in Qt-based mozc_renderer #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

0651fbc

Environment

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