The new page-overlay HTML introduced six CSS classes (.overlay-pick,
.overlay-intro, .overlay-file-row, .overlay-picked, .overlay-hint,
.overlay-skip) plus the .btn-primary anchor and a native checkbox.
None of them had stylesheet rules, so the user saw stacked unstyled
buttons, an OS-native checkbox glyph, and broken alignment.
Adds the missing block under "Overlay step (Flash + Overlay modes)":
- .overlay-pick: vertical layout with consistent gap.
- .overlay-intro: dim 13px paragraph; inline <code> gets a card-coloured
pill so the filename mipi-panel.dtbo stands out.
- .btn-primary: solid blue button variant used by the "Open the Overlay
Generator" anchor.
- .overlay-file-row: flex row gluing the file picker button and the
selected-filename label; the label gets a mono font and wraps.
- .overlay-hint: smaller dim hint above the skip checkbox.
- .overlay-skip: custom checkbox styled to match the wizard (no native
glyph, blue fill + white check when active).
Also normalises the two preexisting var(--font-mono) refs (line 656 and
690) to a real monospace stack, since var(--font-mono) was never
defined in :root and was silently inheriting the sans body font.
Douglas Teles