WebUI Dark Mode: fix active (in progress) download description text color

Screenshots: https://imgur.com/a/VIMVO2d
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c485b44#slide=id.g4a64df24f4_20_0

R=dpapad@chromium.org
BUG=923224

Change-Id: I7292cdcb35b23320f67de75cc6cbbe1b4f2f9003
Reviewed-on: https://chromium-review.googlesource.com/c/1432795
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625742}

chrome/browser/resources/md_downloads/item.html

@@ -198,7 +198,7 @@
         margin-top: 16px;
       }
 
-      .is-active #description {
+      :host-context(html:not([dark])) .is-active #description {
         color: #616161;
       }