The colour of the selection is set by your OS settings by default in TXS.
As of now (TXS version 2.12.8), the option to change the colour of this element is not available in the general settings of TXS, I believe.
The solution now is to use QSS stylesheets, discussed partially in the project Github thread.
Basically, create a file stylesheet.qss, and place it in your TXS settings folder.
Paste this in:
QWidget {
selection-background-color: #db1c49;
selection-color: #eff0f1;
}
and restart TXS.
Note:
Change the colours as you see fit. selection-background-color controls the highlight colour (now red), and selection-color is the colour of the words (now white).
This also affects other elements in TXS, since this affects all QWidgets. There are ways to localize this I'm guessing, but I'm no expert in this regard.

QTextEdit { selection-background-color: rgb(140, 90, 64) }did not work. – Niklas Apr 24 '18 at 03:07QWidget { selection-background-color: #db1c49; selection-color: #eff0f1; }? – Troy Apr 24 '18 at 07:29