Actually, there's example css, plugin css, core css.

src/scss/dragdrop.scss

@@ -0,0 +1,40 @@
+/**
+* Drag & Drop CSS to style the plugin
+*/
+.UppyDragDrop {
+  width: 300px;
+  text-align: center;
+  padding: 100px 10px;
+}
+
+/* http://tympanus.net/codrops/2015/09/15/styling-customizing-file-inputs-smart-way/ */
+.UppyDragDrop-input {
+  width: 0.1px;
+	height: 0.1px;
+	opacity: 0;
+	overflow: hidden;
+	position: absolute;
+	z-index: -1;
+}
+
+.UppyDragDrop.is-dragdrop-supported {
+  border: 2px dashed;
+  border-color: #ccc;
+}
+
+.UppyDragDrop-label {
+  cursor: pointer;
+}
+
+.UppyDragDrop-dragText {
+  display: none;
+}
+
+.is-dragdrop-supported .UppyDragDrop-dragText {
+  display: inline;
+}
+
+.UppyDragDrop.is-dragover {
+  border-color: #d2ecea;
+  background-color: #dbf5f3;
+}

src/scss/uppy.scss

@@ -1,3 +1,9 @@
+/**
+* Uppy CSS and all of its out-of-the-box plugins:
+*/
+
+@import 'dragdrop.scss';
+
 .uppy {
   font-family: "Comic Sans MS";
   color: purple;

website/src/examples/dragdrop/app.css

@@ -1,5 +1,5 @@
 /**
-* Drag & Drop
+* Drag & Drop CSS to style the demo itself
 */
 
 .UppyDragDrop-puppy {
@@ -10,41 +10,3 @@
   display: block;
   margin: 20px 0;
 }
-
-.UppyDragDrop {
-  width: 300px;
-  text-align: center;
-  padding: 100px 10px;
-}
-
-/* http://tympanus.net/codrops/2015/09/15/styling-customizing-file-inputs-smart-way/ */
-.UppyDragDrop-input {
-  width: 0.1px;
-	height: 0.1px;
-	opacity: 0;
-	overflow: hidden;
-	position: absolute;
-	z-index: -1;
-}
-
-.UppyDragDrop.is-dragdrop-supported {
-  border: 2px dashed;
-  border-color: #ccc;
-}
-
-.UppyDragDrop-label {
-  cursor: pointer;
-}
-
-.UppyDragDrop-dragText {
-  display: none;
-}
-
-.is-dragdrop-supported .UppyDragDrop-dragText {
-  display: inline;
-}
-
-.UppyDragDrop.is-dragover {
-  border-color: #d2ecea;
-  background-color: #dbf5f3;
-}

website/themes/uppy/source/css/uppy.css

@@ -1,3 +1,40 @@
+/**
+* Uppy CSS and all of its out-of-the-box plugins:
+*/
+/**
+* Drag & Drop CSS to style the plugin
+*/
+.UppyDragDrop {
+  width: 300px;
+  text-align: center;
+  padding: 100px 10px; }
+
+/* http://tympanus.net/codrops/2015/09/15/styling-customizing-file-inputs-smart-way/ */
+.UppyDragDrop-input {
+  width: 0.1px;
+  height: 0.1px;
+  opacity: 0;
+  overflow: hidden;
+  position: absolute;
+  z-index: -1; }
+
+.UppyDragDrop.is-dragdrop-supported {
+  border: 2px dashed;
+  border-color: #ccc; }
+
+.UppyDragDrop-label {
+  cursor: pointer; }
+
+.UppyDragDrop-dragText {
+  display: none; }
+
+.is-dragdrop-supported .UppyDragDrop-dragText {
+  display: inline; }
+
+.UppyDragDrop.is-dragover {
+  border-color: #d2ecea;
+  background-color: #dbf5f3; }
+
 .uppy {
   font-family: "Comic Sans MS";
   color: purple;