Fix dark theme in file exists dialog

Signed-off-by: szaimen <szaimen@e.mail.de>

apps/files/css/upload.scss

@@ -127,7 +127,7 @@
 }
 .oc-dialog .fileexists th label {
 	font-weight: normal;
-	color:black;
+	color: var(--color-main-text);
 }
 .oc-dialog .fileexists th .count {
 	margin-left: 3px;