css: dead rules out, off-scale values on the scale, undefined classes resolved !224

The stylesheet items from the review (#131).

• Dead rules removed: .mirrorline, .mirrorerr, .railsettings, .container, main.container, .repogrid; a dark block that restated the light value; the second of two form.inline and .repohead .topics declarations (the later one was winning, so it is the one kept).
• On the scale: the landing headline and the 404 code join the type scale as --fs-6 and --fs-7; the dropdown shadow is a --shadow token defined in both schemes; the rail item padding is --sp-1; the blame border uses --r-md instead of an undefined --rad-2 with a 6px fallback that broke the 2px rule.
• Undefined classes: templates asked for .primary on five buttons, which a bare button already is, so the class is gone; .pathbar .actions is what five templates use and the stylesheet spelled .act. The edit form's two inline widths move to rules.

Not touched: the three breakpoints (40, 52, 62rem). Unifying them changes layout and wants a screenshot pass, which belongs with #134.

Stacked on !223; no code dependency.

Closes #131