Remove the dropzone's cream background entirely

Shrinking the box still left a full-width tinted rectangle around the upload
control. Drop its background, border and padding so only the upload button and
the chosen file show. The tint came from the theme's secondaryBackgroundColor,
which expanders also use, so this is scoped to the dropzone rather than changed
in config.toml.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app.py

@@ -220,14 +220,19 @@ st.markdown(
        the padding so the box is the size of what it holds. The dropzone's test
        id was renamed between Streamlit versions, so both spellings are matched
        and whichever one is absent costs nothing. */
-    /* The dropzone's own styles set min-height: 4.25rem on top of 1rem padding,
-       which is what made the control read as a slab. Three things matter when
-       overriding it: it is a <section>, not a <div>; its emotion styles are
-       injected at render time, so !important is needed to win; and the tag has
-       changed across versions, so match on the test id alone. */
+    /* Strip the dropzone's cream slab entirely: no background, no border, no
+       padding, so what shows is the upload button and, once chosen, the file
+       itself. Three things matter when overriding it: it is a <section>, not a
+       <div>; its emotion styles are injected at render time, so !important is
+       needed to win; and the tag has changed across versions, so match on the
+       test id alone. The backgroundColor it used is the theme's
+       secondaryBackgroundColor, which expanders still use, so this is scoped to
+       the dropzone rather than changed in config.toml. */
     [data-testid="stFileUploaderDropzone"] {
+        background: transparent !important;
+        border: none !important;
         min-height: 0 !important;
-        padding: .45rem .7rem !important;
+        padding: 0 !important;
         gap: .7rem !important;
         align-items: center !important;
     }