Update See @Jakub Zawiślak's answer for VScode 1.12+
Old answer
Visual Studio Code calls this selection highlighting and unfortunately, I don't think the color is customizable currently. Themes can control the 'selection' color, but the 'selection highlight' color is hardcoded.
See this issue tracking a possible solution: https://github.com/Microsoft/vscode/issues/1636
(As a side note, you can toggle this feature or/off with the editor.selectionHighlight
setting.)