Range control style issues

[yuri-tceretian]

1. The range control looks disabled when it is used in a layout with more contrast controls (screenshot below).
   
   During testing Elastic Cloud Admin UI (part of it in the screenshot) I caught myself trying to find some check\radio box to enable a slider because it looked like disabled. The control wasn't disabled but controls around it made me feel like it was. I think some contrast border for the circle handle would be great to avoid such confusion.

2. Labels for the range control are barely readable in dark mode (see screenshot below)
   

[cchaos]

The styling of the range control has been fixed in #932. That should be sufficient to close this ticket. I've made a separate ticket (#960) for creating a more customizable range control like the one used in these screenshots.