593 More-on-CSS-Variables
https://developer.mozilla.org/en-US/docs/Web/CSS/--*

603 Dive-into-CSS-Flexbox
https://academind.com/learn/css/understanding-css/flexbox-basics-container/

612 More-about-CSS-Transitions
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Transitions/Using_CSS_transitions