Add matching color for text selection on slides

[joberkel]

When the mousepointer is used to select/highlight some text on the slides, the default color of the selection makes the text less readable.

--> change the color for a better visibility when highlighting. Focus on text, but also try out on headlines which have a different color.
(see implementation of css: https://www.w3schools.com/howto/howto_css_text_selection.asp )