Fix check style in poll option

Author: langleyd

res/css/components/views/polls/_PollOption.pcss

@@ -59,13 +59,25 @@ Please see LICENSE files in the repository root for full details.
     /* override checked radio button styling to show checkmark instead */
     .mx_StyledRadioButton_checked {
         input[type="radio"]:checked + div {
+            position: relative;
             border-width: 2px;
             border-color: var(--cpd-color-icon-primary);
             background-color: var(--cpd-color-icon-primary);
-            background-image: url("@vector-im/compound-design-tokens/icons/check.svg");
-            background-size: 12px;
-            background-repeat: no-repeat;
-            background-position: center;
+
+            &::before {
+                content: "";
+                position: absolute;
+                top: 50%;
+                left: 50%;
+                transform: translate(-50%, -50%);
+                mask-image: url("@vector-im/compound-design-tokens/icons/check.svg");
+                mask-size: 12px 12px;
+                mask-repeat: no-repeat;
+                mask-position: center;
+                background-color: var(--cpd-color-icon-on-solid-primary);
+                width: 12px;
+                height: 12px;
+            }
 
             div {
                 visibility: hidden;