Added keyboard tab navigation.
Selected item/element is highlighted with an orange outline/border.
All elements should be accessible via keyboard (except for the svg-icon at the moment, still in progress)

Changes:
- Highlight-Color changed
- every element should now be reachable and selectable/clickable via keyboard
- added a few tabindex=-1 for elements, that should not be reachable via keyboard (and are also not clickable via mouse)

- removed box-shadow
- adjusted colors for dark and light background
- adjusted colors for buttons

Logo is now accessible via keyboard.
Small bug fixed (navigation bar could be controlled via keyboard during recording.)

- removed background-color from global-style.js
- added '&:not(:disabled):focus' to buttons and text in theme.js