Updated Dogcom logo for better light mode support. (#3472)

* Updated dogcom logo (for better light mode support).
* Invertable class removed inadvertently
* Missed `sponsor-img` class.
* Use underscore to stay inline with current standard

Author: blckmn

src/css/dark-theme.less

@@ -941,3 +941,6 @@ progress[value] {
 		}
 	}
 }
+.invertable {
+	filter: invert(1)
+}

src/css/main.less

@@ -1161,6 +1161,9 @@ dialog {
 	max-height: 50px;
 	margin: 0 auto 10px auto
 }
+.img_sponsor {
+	height: 100%
+}
 .note {
 	background-color: #fff7cd;
 	border: 1px solid #ffe55f;