fix focus highlight on bright theme

src/skins/vector/css/matrix-react-sdk/views/elements/_AccessibleButton.scss

@@ -16,6 +16,6 @@ limitations under the License.
 
 .mx_AccessibleButton:focus {
     outline: 0;
-    filter: brightness(200%);
+    filter: brightness($focus-brightness);
 }
 

src/skins/vector/css/themes/_base.scss

@@ -15,6 +15,8 @@ $accent-color: #76CFA6;
 
 $selection-fg-color: $primary-bg-color;
 
+$focus-brightness: 125%;
+
 // red warning colour
 $warning-color: #ff0064;
 

src/skins/vector/css/themes/_dark.scss

@@ -15,6 +15,8 @@ $accent-color: #76CFA6;
 
 $selection-fg-color: $primary-fg-color;
 
+$focus-brightness: 200%;
+
 // red warning colour
 $warning-color: #ff0064;