osd-classic: add theme options for selected window item

osd-classic: add theme options for selected window item

This commit adds new theme options:
- osd.window-switcher.style-classic.item.active.border.color
- osd.window-switcher.style-classic.item.active.bg.color

These theme options configures the border/background of selected window
item in the `classic` style window switcher. Their default values are
identical to `thumbnail` style window switcher, which means the default
border color is now `osd.label.text.color` with 50% opacity and the
default background color is now `osd.label.text.color` with 15% opacity.