Restyled the butter bar in Files.app.

chrome/browser/resources/file_manager/css/common.css

Index: chrome/browser/resources/file_manager/css/common.css
diff --git a/chrome/browser/resources/file_manager/css/common.css b/chrome/browser/resources/file_manager/css/common.css
index d94a533533c64fddedbe44b1e1ebb740160a259f..83d29d292fb60e8dd0eb7c9c15e6bffdea659ac5 100644
--- a/chrome/browser/resources/file_manager/css/common.css
+++ b/chrome/browser/resources/file_manager/css/common.css
@@ -380,7 +380,7 @@ input[type='submit'][disabled]:hover {
 }
 
 /* Blue progress bar. */
-.progress-bar {
+body:not([new-ui]) .progress-bar {
   background-color: #e6e6e6;
   border: 1px solid #ddd;
   border-radius: 2px;
@@ -388,7 +388,7 @@ input[type='submit'][disabled]:hover {
   margin: 2px 0;
 }
 
-.progress-track {
+body:not([new-ui]) .progress-track {
   background-image: -webkit-linear-gradient(
       rgb(110, 188, 249) 1px, rgb(86, 167, 247) 2px,
       rgb(76, 148, 245) 11px, rgb(41, 112, 211) 12px);
@@ -398,6 +398,18 @@ input[type='submit'][disabled]:hover {
   margin: -1px;
 }
 
+body[new-ui] .progress-bar {
+  background-color: #e6e6e6;
+  border-radius: 3px;
+  height: 6px;
+}
+
+body[new-ui] .progress-track {
+  background-color: #888;
+  border-radius: 3px;
+  height: 6px;
+}
+
 .progress-track.smoothed {
   -webkit-transition: width 1s linear;
 }