Replace Sass variables with custom properties on _font-weights.pcss (#10554)

* Replace `$font-normal` and `$font-semi-bold`

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Replace `font-weight: 600` with the custom property

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

* Replace the existing variables

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

---------

Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>

res/css/structures/_RoomSearch.pcss

@@ -51,7 +51,7 @@ limitations under the License.
         /* the following rules are to match that of a real input field */
         overflow: hidden;
         margin: 9px;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
     }
 
     .mx_RoomSearch_shortcutPrompt {
@@ -62,7 +62,7 @@ limitations under the License.
         font-size: $font-12px;
         line-height: $font-15px;
         font-family: inherit;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         color: $light-fg-color;
         margin-right: 6px;
         white-space: nowrap;