[3.0] Theme split (wave 4, part 26) — point the drop menu submenus at design tokens

[albertlast]

Description

Part of the #7933 split, and the other half of #9467. That one did the level 1 buttons; this does the level 2 and 3 submenu wrapper, the items inside it, and their hover state — the rest of the hard-coded colours in the drop menu rules.

20 tokens in /** Drop Menu Submenus **/ and /** Drop Menu Submenu Items **/. Every token holds the value the rule has today, so nothing renders differently.

The wrapper deliberately sets a border and then overrides the left and top edges with lighter colours:

border: var(--submenu-border-style) var(--submenu-border-width) var(--submenu-border-color);
border-left: var(--submenu-border-style) var(--submenu-border-width) var(--submenu-border-color_left);
border-top: var(--submenu-border-style) var(--submenu-border-width) var(--submenu-border-color_top);

so those edges keep their own tokens rather than being flattened, and all three stay shorthands. .dropmenu li li a:hover does the same thing and is treated the same way.

Verification

0 differences, by two independent methods:

• Real DOM: a ul.dropmenu > li > ul > li > a built on the board index, 5 elements, 32 computed longhands each — 160 values, 0 differences.
• Raw source: index.css fetched over HTTP and its rule bodies applied to a probe in cascade order, to reach the :hover states a script cannot trigger — 160 values, 0 differences.

The second method exists because of a trap worth flagging for anyone reviewing this kind of change. Reading rule text back out of CSSRule.style.cssText and replaying it cannot round-trip a border shorthand that holds a var() and is then overridden on one edge. Doing it that way reported 21 confident-looking differences here — right and bottom borders collapsing to 0px none currentColor — none of which are real. The rule text has to come from the file, or the measurement has to be taken on a real element.

This touches variables.css at the same place as #9467, so whichever lands second needs a trivial rebase. Both insert next to /** Main Menu **/ rather than at the end of the file, so neither collides with #9443 or #9448.

Issues References (Fixes|Related|Closes)

Related: #7933, #9467