viewer.css: GEM APV "Live" button unreadable in light theme (undefined --bg-dim/--bg-hover); eff cards use undefined --canvas

[Chao1009]

Where

• APV toolbar button: https://github.com/JeffersonLab/prad2evviewer/blob/d21ae9f/resources/viewer.css#L588-L591
• Efficiency z-histogram and cards: https://github.com/JeffersonLab/prad2evviewer/blob/d21ae9f/resources/viewer.css#L539-L549
• Theme tokens that should be used instead: https://github.com/JeffersonLab/prad2evviewer/blob/d21ae9f/resources/viewer.css#L100-L111

What happens

--bg-dim, --bg-hover and --canvas are not defined anywhere. They are missing from all three theme blocks and from the legacy alias list in viewer.css, and theme.js does not set them either.

• .gem-apv-toolbar button therefore always uses the hard-coded dark fallbacks #1a1a2a (normal) and #22223a (hover), whatever the active theme. Its text color is var(--text), which the light theme sets to #1d1d1f. In the light theme the only toolbar button (#gem-apv-pause, "▶ Live") is near-black text on a near-black background. The contrast ratio is about 1.02:1, or 1.09:1 on hover, so the label is effectively invisible. The paused state is not affected because it overrides the background with --accent.
• .gem-eff-zhist and .gem-eff-card use background:var(--canvas) with no fallback. That declaration is invalid at computed-value time, so the background becomes transparent in every theme. The efficiency cards and z-histogram frame never get the intended --theme-canvas shade. This part is cosmetic only.

Evidence

/* viewer.css @ d21ae9f */
.gem-eff-zhist { ... background:var(--canvas); }            /* L542 */
.gem-eff-card  { ... background:var(--canvas); }            /* L548 */
.gem-apv-toolbar button { ...
    background:var(--bg-dim,#1a1a2a); color:var(--text); ... }  /* L589 */
.gem-apv-toolbar button:hover { background:var(--bg-hover,#22223a); }  /* L591 */

git grep -e '--bg-dim' -e '--bg-hover' -e 'var(--canvas' d21ae9f -- resources/ only finds these uses and no definitions.

To reproduce: click the header theme button (#btn-theme) until the theme is "light", then open the GEM APV tab. The "▶ Live" button reads as a dark block with no visible label.

Suggested fix

Use the theme tokens that already exist, since each theme defines them:

.gem-eff-zhist, .gem-eff-card { background:var(--theme-canvas); }
.gem-apv-toolbar button       { background:var(--theme-button); }
.gem-apv-toolbar button:hover { background:var(--theme-button-hover); }

Alternatively, add --canvas: var(--theme-canvas); to the legacy alias block. After the change, check the cards and histogram in all three themes, because their shade will change slightly.

Status

Still present on dedup-refactor. There the undefined vars were replaced by the values they fell back to: hard-coded #1a1a2a/#22223a, and the background line removed from .gem-eff-zhist/.gem-eff-card. Rendering is unchanged, so the light-theme button is still unreadable.