admin(typography): clean up h3 rules superseded by unified global heading

Follow-up to the previous commit that unified .admin-main h1/h2/h3 to
the 28px page-title scale. Audit pass over the existing rules to
remove duplicated work and per-page sizing overrides that now defeat
the global intent.

Trimmed redundant properties (kept the rule for its unique work):
  - sidebar-nav.css '.admin-main .content-header h3' — dropped color
    / font-size / font-weight / line-height. Kept margin:0 (wrapper
    handles spacing), padding:0, boxes.css background wipe, overflow
    ellipsis.
  - sidebar-nav.css second '.content-header h3, .head-products, ...'
    block — dropped h3 from the selector list, kept the legacy span
    selectors (.head-products / .head-sales-order / .head-customer)
    that still need explicit 28px since they're <span>s.
  - dark-theme.css '.content-header h3' — dropped font-size:18px /
    font-weight / letter-spacing / text-transform / margin /
    line-height. Kept color (defensive outside .admin-main), boxes.css
    background wipe, padding-left:0.

Removed entirely (per-page h3 size overrides that contradicted the
unified 28px scale or were pure redundancy):
  - body.adminhtml-permissions-user-(edit|new) .content-header h3
    { font-size:20px }
  - body[class*='adminhtml-system_config'] .content-header h3
    { font-size:18px }
  - body[class*='customoptions-options-edit'] .content-header h3
    portion — split so only the h4 sub-label rule remains.
  - .dashboard .box-head h3 { color:var(--t1) } — single-property
    rule, color is now identical to what global emits.

Left intentionally:
  - cache-index / process-list h3 — those h3s are deliberate small
    uppercase section labels (e.g. 'Additional Cache Management'),
    not page titles, so the per-page small override is the correct
    design role.
  - catalog-product-edit .side-col h3 — sidebar tab labels.
  - .admin-main .box-head h3 — panel section headers.
  - workflowguides .wfg-guide-header h2 — sets display:flex layout,
    not just sizing.
  - is-order-view / catalog-category side-col / generic
    h1-h6 { color:var(--t1) } — defensive coverage for headings
    outside .admin-main (modals, popups).

Author: unknown

skin/adminhtml/default/default/dark-theme.css

@@ -386,16 +386,15 @@ body .header-top {
     padding: 0 !important;
 }
 .content-header h3, .content-header h3.icon-head {
+    /* color / font-size / font-weight / letter-spacing / text-transform
+       / margin now come from the unified global heading rule in
+       sidebar-nav.css. Keep only the legacy-boxes.css icon-background
+       wipe and the zero left-padding so this rule still earns its keep
+       outside of .admin-main (modals, side panels, etc.). */
     color: var(--t1) !important;
     background: none !important;
     background-image: none !important;
-    font-size: 18px !important;
-    font-weight: 700 !important;
-    letter-spacing: -0.1px !important;
-    text-transform: none !important;
     padding-left: 0 !important;
-    margin: 0 !important;
-    line-height: 1.2 !important;
 }
 /* Page container */
 #page\:main-container { padding: 16px 10px 16px 8px !important; }
@@ -3086,7 +3085,8 @@ ul.tabs-horiz li a.active {
     background-image: none !important;
     border-color: var(--b1) !important;
 }
-.dashboard .box-head h3 { color: var(--t1) !important; }
+/* `.dashboard .box-head h3` color rule removed — color now comes from the
+   unified global heading rule (also var(--t1)). */
 .dashboard .col2-set, .dashboard .col2-set .col-1, .dashboard .col2-set .col-2 { background: var(--d1) !important; }
 .dashboard .col-2 .box, .dashboard .col-2 .box .hor-scroll, .dashboard .col-2 .box .box-head, .dashboard .col-2 { background: var(--d3) !important; }
 .dashboard .box .totals, .dashboard .box .totals table,
@@ -3690,12 +3690,9 @@ body.adminhtml-permissions-user-new #edit_form hr {
     display: none !important;
 }
 
-/* Header row: title + button bar. Buttons get the "modal footer" treatment. */
-body.adminhtml-permissions-user-edit .content-header h3,
-body.adminhtml-permissions-user-new .content-header h3 {
-    font-size: 20px !important;
-    font-weight: 700 !important;
-}
+/* Header row: title + button bar. Buttons get the "modal footer" treatment.
+   (Per-page h3 size override removed — the unified global heading rule in
+   sidebar-nav.css now sets every h1/h2/h3 to 28px / 700 consistently.) */
 body.adminhtml-permissions-user-edit .content-header button.back,
 body.adminhtml-permissions-user-new .content-header button.back {
     background: transparent !important;
@@ -3865,13 +3862,8 @@ body[class*="adminhtml-system_config"] .content-header {
     margin: 0 0 4px !important;
 }
 body[class*="adminhtml-system_config"] .content-header > table { width: 100% !important; }
-body[class*="adminhtml-system_config"] .content-header h3 {
-    color: var(--t1) !important;
-    font: 700 18px/1.2 inherit !important;
-    text-transform: none !important;
-    letter-spacing: -0.1px !important;
-    margin: 0 !important;
-}
+/* Per-page h3 size override removed — unified by the global rule in
+   sidebar-nav.css ('.admin-main h1, h2, h3' → 28px / 700). */
 
 /* Section row — no card. Hairline between siblings. Kill ALL wrappers. */
 body[class*="adminhtml-system_config"] #config_edit_form .entry-edit,
@@ -4924,8 +4916,9 @@ body[class*="customoptions-options-edit"] [id*="group"][class*="box"] {
     color: var(--t2) !important;
 }
 
-/* Compact page header (Edit Template '...') */
-body[class*="customoptions-options-edit"] .content-header h3,
+/* Compact page header (Edit Template '...') — h3 now follows the global
+   28px rule; h4 keeps its compact 13px override here since it's used as
+   a sub-label on this page. */
 body[class*="customoptions-options-edit"] .content-header h4 {
     font-size: 18px !important;
     font-weight: 700 !important;

skin/adminhtml/default/default/sidebar-nav.css

@@ -867,16 +867,17 @@ body.sidebar-collapsed .admin-main {
 .admin-main .content-header td[style*="float"] {
     float: none !important;
 }
+/* Color / font-size / font-weight / line-height now come from the
+   unified global heading rule at the bottom of this file. Keep only
+   the properties this rule was uniquely contributing: zero out the
+   margin/padding so the .content-header wrapper handles spacing, wipe
+   the legacy boxes.css icon backgrounds, and let long titles ellipsis. */
 .admin-main .content-header h3,
 .admin-main .content-header h3.icon-head {
-    color: var(--t1) !important;
-    font-size: 28px !important;
-    font-weight: 700 !important;
     margin: 0 !important;
     padding: 0 !important;
     background: none !important;
     background-image: none !important;
-    line-height: 1.2 !important;
     white-space: nowrap;
     overflow: hidden;
     text-overflow: ellipsis;
@@ -1259,11 +1260,11 @@ body table.data tr.selected td a {
     color: #64748b !important;
 }
 
-/* Page title — unified canonical size (no glow). 28px / 700.
-   See admin-dashboard.css → "Global page-title type ramp" for the
-   single source of truth that also normalises every MMD panel
-   `<h1 class="*-title">`. */
-.admin-main .content-header h3,
+/* Page title spans inside .content-header — the legacy boxes.css
+   ships .head-products / .head-sales-order / .head-customer as <span>
+   classes used as icon-headers. Headings are normalised by the unified
+   global heading rule at the bottom of this file; these span selectors
+   still need the title scale since they aren't h1/h2/h3 elements. */
 .admin-main .content-header .head-products,
 .admin-main .content-header .head-sales-order,
 .admin-main .content-header .head-customer {