stylesheet improvements; use data-lang for source listings

- lighten bold where Open Sans is used (headings, verses)
- add border-collapse: separate where borders are used on tables
- remove unnecessary font-weight declarations
- use Open Sans for admonition label
- set padding bottom on paragraph in rems
- use 1.45 for sidebarblock line-height
- simplify the selector to set background-color on listing & literal blocks
- use data-lang to create language hover text for any language on listing blocks
- use border: none instead of border: 0
remove space before !important; switch font-weight 700 to bold