diff --git a/ZB.MOM.WW.Theme/README.md b/ZB.MOM.WW.Theme/README.md
index 6ef8a2b..a5747ba 100644
--- a/ZB.MOM.WW.Theme/README.md
+++ b/ZB.MOM.WW.Theme/README.md
@@ -80,7 +80,15 @@ Served at `_content/ZB.MOM.WW.Theme/…` by the ASP.NET static-web-asset pipelin
 | Path | Contents |
 |---|---|
 | `css/theme.css` | Design tokens (`--accent`, `--ok`, `--warn`, …), typography, utility helpers |
-| `css/layout.css` | Side-rail shell layout, collapsible nav, `StatusPill` variants, card/field helpers |
+| `css/layout.css` | Side-rail shell layout, collapsible nav, `StatusPill` variants, card/field helpers, Bootstrap `.btn` sizing |
+
+**Bootstrap button sizing (new in 0.4.0).** The kit now normalizes `.btn` / `.btn-sm` against its
+0.9rem body via `--bs-btn-*` variable overrides, because Bootstrap hard-codes
+`--bs-btn-font-size: 1rem`. All five consuming apps had written this same block into their own
+stylesheets; **delete the local copy when you bump to 0.4.0** (each carries a comment saying so).
+Size buttons by overriding the `--bs-btn-*` variables, never by redeclaring box properties — the
+`.btn-group` seam and radius machinery reads those variables, so a literal `font-size` at equal
+specificity silently reduces `.btn-sm` to a padding-only difference.
 
 Rail-footer actions: `class="rail-btn"` is the inline default; add `rail-btn-block`
 (`class="rail-btn rail-btn-block"`) for a full-width footer button. Use the modifier rather than
diff --git a/ZB.MOM.WW.Theme/src/ZB.MOM.WW.Theme/wwwroot/css/layout.css b/ZB.MOM.WW.Theme/src/ZB.MOM.WW.Theme/wwwroot/css/layout.css
index ef18046..f5b0509 100644
--- a/ZB.MOM.WW.Theme/src/ZB.MOM.WW.Theme/wwwroot/css/layout.css
+++ b/ZB.MOM.WW.Theme/src/ZB.MOM.WW.Theme/wwwroot/css/layout.css
@@ -1,6 +1,34 @@
 /* ZB.MOM.WW.Theme — side-rail + login layout
    Tokens live in theme.css; this sheet carries only layout + the side rail. */
 
+/* ── Bootstrap button sizing ─────────────────────────────────────────────────
+   Bootstrap hard-codes --bs-btn-font-size: 1rem, which is oversized against
+   this kit's 0.9rem body. Every consuming app had independently written this
+   same block into its own stylesheet (ignitionoee app.css, OtOpcUa site.css,
+   mxgw site.css, HistorianGateway site.css, ScadaBridge site.css) — it is
+   upstreamed here in 0.4.0, and those local copies can be deleted on bump.
+
+   Sized by CSS-variable override ONLY — never by redeclaring box properties.
+   Bootstrap's .btn-group seam and radius machinery reads these same variables,
+   so a literal `font-size` here at equal specificity would flatten .btn-sm into
+   a no-op that differs only in padding. That was a real defect found in mxgw's
+   local copy; do not "simplify" this back into literals.
+
+   ThemeHead emits this sheet after theme.css and both after Bootstrap, so the
+   override wins on source order at equal specificity. */
+.btn {
+    --bs-btn-font-size: .85rem;
+    --bs-btn-padding-y: .35rem;
+    --bs-btn-padding-x: .8rem;
+    --bs-btn-line-height: 1.4;
+}
+.btn-group-sm > .btn,
+.btn-sm {
+    --bs-btn-font-size: .78rem;
+    --bs-btn-padding-y: .25rem;
+    --bs-btn-padding-x: .6rem;
+}
+
 /* ── App shell: side rail + page ─────────────────────────────────────────── */
 /* The shell is a native <details> disclosure so the narrow-viewport hamburger
    toggle works with NO JavaScript (no Bootstrap collapse bundle): below lg the