osd-thumbnail: update default colors of selected window item

osd-thumbnail: update default colors of selected window item

Previously, the default values of
`osd.window-switcher.style-thumbnail.item.active.{bg,border}.color`
were blue. But they caused the selected window title in the window
switcher to be unreadable due to duplicated colors of the text and
background with Openbox themes like Numix.

Instead, this commit updates them to follow other themes configurations.
The default border color of the selected window item is now
`osd.label.text.color` with 50% opacity and the background is
`osd.label.text.color` with 15% opacity.

For subpixel antialiasing to work, the background color is calculated by
manually blending `osd.label.text.color` and `osd.bg.color`, rather than
just updating the alpha with 50% or 15%.