[Minor] Rename `colors` variable to `theme` in `useTheme()`

[sundaramramaswamy]

This came-up in code review of #654. We want to follow explicit naming for better readability. All instances of

  const {colors} = useTheme();
  // later usage
  colors.border

should be renamed to

  const {theme} = useTheme();
  // later usage
  theme.border

There are too many instances to take it up as part of #654 so creating a separate bug.