Make DOM panel look more like the DevTools when switching to light or dark theme

[SebastianZ]

When switching to the light or dark theme, the DOM panel display should be adjusted to fit to the style used by the DevTools.

Tasks:

• [ ] Adjust the display of the Refresh toolbar button (hover and non-hover state)
• [ ] Adjust the font within the list
• [ ] Adjust the colors within the list to fit to the ones used within the variables view
• [ ] Replace the +/- twisties by arrows
• [ ] Replace the lock icon by one that fits the general icon style (flat, monochrome)
• [ ] Alter the background of the list items (like within the Network panel)

Sebastian