diff --git a/assets/checksums.txt b/assets/checksums.txt
index 099069c5c..c9d9290a2 100644
--- a/assets/checksums.txt
+++ b/assets/checksums.txt
@@ -1,7 +1,7 @@
 2d0a4786a7175bf73b788436cf65bc5a assets/ublock/filter-lists.json
 6572a7bce5f1747fee597cb170bd3b98 assets/ublock/privacy.txt
 6fdc6f9d056ebb56b6fe1a1db1173a94 assets/ublock/filters.txt
-306e54c080ce68afe7ae7cef9a2541e8 assets/ublock/mirror-candidates.txt
+4d9e52dd0f2fa7b4ff3c80043096e542 assets/ublock/mirror-candidates.txt
 36a285b3ef9aadad4529d40153364013 assets/thirdparties/adblock-chinalist.googlecode.com/svn/trunk/adblock.txt
 d8c58f5bb8072907ae361bc14dbbd28f assets/thirdparties/cjxlist1.googlecode.com/svn/cjxlist.txt
 132b3ecc9da8a68c3faf740c00af734b assets/thirdparties/adblock-plus-japanese-filter.googlecode.com/hg/abp_jp.txt
diff --git a/assets/ublock/mirror-candidates.txt b/assets/ublock/mirror-candidates.txt
index dc7067313..674831478 100644
--- a/assets/ublock/mirror-candidates.txt
+++ b/assets/ublock/mirror-candidates.txt
@@ -30,7 +30,9 @@ platform.twitter.com
 # https://github.com/gorhill/uBlock/issues/270#issuecomment-57913228
 # Ref: http://www.asp.net/ajax/cdn
 ajax.aspnetcdn.com
-    ^ajax\.aspnetcdn\.com\/ajax\/[^?#]+
+    # Avoid CSS-based themes, because relative URLs won't work from
+    # within a data: URI
+    ^ajax\.aspnetcdn\.com\/ajax\/(?!.*\/themes\/.+\.css)[^?#]+
 
 # Ref: https://tech.yandex.ru/jslibs/
 yastatic.net