Use proper color for highlighted text in log widget

[Chocobo1]

• Revive dark theme detection The code was removed in 199d770 and now revived. I reckon this function would be useful in the future despite it isn't in use currently.
• Simplify code Don't use replacement placeholder and use string append which is faster.
• Use proper color for highlighted text in log widget The color is either from qbt theme pack or desktop environment. Before: After:

[glassez]

• The color is either from qbt theme pack or desktop environment.

I wonder how it looks like with some custom theme.

[Chocobo1]

I wonder how it looks like with some custom theme.

It will use color from custom theme: https://github.com/qbittorrent/qBittorrent/blob/master/src/gui/uithememanager.cpp#L336

It is the same (highlighted text) color you see in torrent transfer list.

[xavier2k6]

@Chocobo1 Try running this PR as you would normally on windows & you will get below when selecting! (not so easy on the eyes/readable!)

One way around this is to run qBittorrent under fusion style

[Chocobo1]

@Chocobo1 Try running this PR as you would normally on windows & you will get below when selecting! (not so easy on the eyes/readable!)

OK I updated the PR. I've done some testing on linux & windows with both native theme and themes from https://github.com/jagannatharjun/qbt-theme. I've found that Windows native theme (and mumble-lite) are the outliers. All other themes would look better with highlighted text color. As for mumble-lite theme, it is fixable by altering the highlight bar color (or highlighted text color) so I won't worry too much about it.

@qbittorrent/bug-handlers Please test it too.

[thalieht]

Looks ok to me everywhere (didn't test all themes from https://github.com/jagannatharjun/qbt-theme nor any Windows high contrast themes).