luci-theme-openwrt-2020: add styling of abbreviations and acronyms

Style abbreviations and acronyms with dotted underline text-decoration
as without the style it's quite hard to find out which text is
abbreviation or acronym.

References: https://developer.mozilla.org/en-US/docs/Archive/Web_Standards/Styling_Abbreviations_and_Acronyms
Signed-off-by: Petr Štetiar <ynezz@true.cz>