Render the script surfaces correctly in dark mode

The explainer panel used var(--surface, #fff) and PCDetail's Run Scripts
and Corporate Tools styles used --text-primary, --text-secondary and
--bg-secondary. None of those four tokens exist in this app's theme, so
var() fell through: the explainer rendered a solid white block over the
dark background and the surrounding copy lost its intended contrast.

The real tokens are --bg-card, --text and --text-light. A fallback
colour is what made this hard to catch - it looks correct in light mode
and only misrenders on the theme the shop floor actually runs, so it
would not have shown up in authoring or in review.

Both files now carry a test asserting their styles reference only
defined tokens and contain no var(--x, #colour) fallbacks. That test
immediately caught a further dead --warning fallback in the banner.

The two long per-field explanations in the Add Script form are now
collapsed behind "How this works". The modal already scrolled, and
three paragraphs of standing prose between the inputs pushed Save off
screen, so the copy was being skimmed past rather than read.

plugins/computers/frontend/views/pcDetailRunScripts.spec.js

@@ -82,4 +82,14 @@ describe('PCDetail Run Scripts tab', () => {
     const handler = source.slice(source.indexOf('async function confirmRunScript'))
     expect(handler.slice(0, handler.indexOf('\n}\n'))).toMatch(/queued for/)
   })
+
+  it('styles the Run Scripts tab from defined theme tokens only', () => {
+    // --text-primary/--text-secondary/--bg-secondary are not defined by this
+    // app's theme. var() then falls through to the fallback, or to inherit,
+    // which reads fine in light mode and misrenders for dark-mode users - the
+    // failure only shows up on the theme most of the shop floor runs.
+    const runscripts = source.slice(source.indexOf('.corptools-intro'))
+    expect(runscripts).not.toMatch(/--text-primary|--text-secondary|--bg-secondary|--surface/)
+    expect(runscripts).not.toMatch(/var\(--[a-z-]+,\s*#/)
+  })
 })