Add .css styling

[JDuchniewicz]

The aim is to have the theme matching the one of browser. Additionally:

• [x] Input box always focused
• [x] No dropdown from input box
• [x] Highlight for currently selected option
• [x] Autoupdate of highlighted option
• [ ] Do a borderless window

[Willmish]

No dropdown and autofocus added as attributes to the input in the HTML file

[JDuchniewicz]

• [x] Colouring of an item should span whole window
• [ ] Remove borders from the window (more immersive feeling)
• [x] Remove title
• [x] Adjust the input field for a more immersive feeling