diff --git a/inst/reveal.js-5.2.1/css/theme/source/ids.scss b/inst/reveal.js-5.2.1/css/theme/source/ids.scss
index e833b8f..f431662 100644
--- a/inst/reveal.js-5.2.1/css/theme/source/ids.scss
+++ b/inst/reveal.js-5.2.1/css/theme/source/ids.scss
@@ -366,6 +366,44 @@
   background: #{$idsOrange} !important;
 }
 
+// Adaptive vertical centering of slide content: the template groups each
+// slide's body content (everything after the heading block except speaker
+// notes) into a div.ids-slide-content. Slides carrying such a wrapper become
+// flex columns of full slide height: the heading block stays pinned to the
+// top and the wrapper's auto margins center the content in the remaining
+// space. Auto margins collapse to zero when there is no free space, so
+// packed slides still start directly below the heading and overflow
+// downwards as before -- the drawback that makes reveal.js' own `center`
+// option unusable here (it would also center the headings). The wrapper
+// keeps the footer band free (--ids-footer-reserve, published by the
+// template from the actual footer height), so centered content never
+// covers the footer; slides too packed to fit above it may, as they always
+// could. Display only ever changes for slides reveal.js is actually
+// showing, mirroring the section-divider rule above, which works the same
+// way; screen-only, so the pdf export keeps laying out slides itself.
+@media screen {
+  html .reveal .slides section:not(.stack):not(.title-frame) {
+    height: 100% !important;
+    display: flex !important;
+    flex-direction: column !important;
+  }
+
+  .reveal .slides section:not(.stack):not(.title-frame) > .ids-slide-content {
+    flex: none;
+    margin-top: auto !important;
+    margin-bottom: auto !important;
+    padding-bottom: var(--ids-footer-reserve, 48px);
+
+    // under block layout the first content element's margin-top collapsed
+    // with the heading's margin-bottom; as flex items the two would add up
+    // instead, so drop it and let the heading spacing rule alone define
+    // the gap, as before
+    > :first-child {
+      margin-top: 0 !important;
+    }
+  }
+}
+
 .reveal .slides section.level1:not(.title-frame) > h1:first-child {
   text-align: center !important;
   margin: 0 !important;
diff --git a/inst/reveal.js-5.2.1/dist/theme/ids.css b/inst/reveal.js-5.2.1/dist/theme/ids.css
index dd54e86..8d56808 100644
--- a/inst/reveal.js-5.2.1/dist/theme/ids.css
+++ b/inst/reveal.js-5.2.1/dist/theme/ids.css
@@ -523,6 +523,40 @@
   content: none !important;
 }
 
+/* Adaptive vertical centering of slide content: the template groups each
+   slide's body content (everything after the heading block except speaker
+   notes) into a div.ids-slide-content. Slides carrying such a wrapper become
+   flex columns of full slide height: the heading block stays pinned to the
+   top and the wrapper's auto margins center the content in the remaining
+   space. Auto margins collapse to zero when there is no free space, so
+   packed slides still start directly below the heading and overflow
+   downwards as before -- the drawback that makes reveal.js' own `center`
+   option unusable here (it would also center the headings). The wrapper
+   keeps the footer band free (--ids-footer-reserve, published by the
+   template from the actual footer height), so centered content never
+   covers the footer; slides too packed to fit above it may, as they always
+   could. Display only ever changes for slides reveal.js is actually
+   showing, mirroring the section-divider rule above, which works the same
+   way; screen-only, so the pdf export keeps laying out slides itself. */
+@media screen {
+  html .reveal .slides section:not(.stack):not(.title-frame) {
+    height: 100% !important;
+    display: flex !important;
+    flex-direction: column !important;
+  }
+
+  .reveal .slides section:not(.stack):not(.title-frame) > .ids-slide-content {
+    flex: none;
+    margin-top: auto !important;
+    margin-bottom: auto !important;
+    padding-bottom: var(--ids-footer-reserve, 48px);
+  }
+
+  .reveal .slides section:not(.stack):not(.title-frame) > .ids-slide-content > :first-child {
+    margin-top: 0 !important;
+  }
+}
+
 :root {
   --r-bold-color: #f6a800;
   --r-list-bullet-color: #f6a800;