[Tumbleweed] Make title QToolButton backgrounds transparent

[dgdavid]

Problem

The unstyled QToolButton holding the theme switcher is not honoring the transparent background used by the PNG file.

Solution

Set the #LogoHBox QToolButton background to transparent.

Screenshots

Before	After

Related links

• https://github.com/yast/yast-theme/pull/156
• https://github.com/openSUSE/branding/pull/133
• https://github.com/libyui/libyui/pull/65