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/_SpacePanel.pcss

@@ -276,7 +276,7 @@ $activeBorderColor: $primary-content;
             border-radius: 8px;
             background-color: $panel-actions;
             font-size: $font-15px !important; /* override inline style */
-            font-weight: $font-semi-bold;
+            font-weight: var(--font-semi-bold);
             line-height: $font-18px;
 
             & + .mx_BaseAvatar_image {
@@ -380,7 +380,7 @@ $activeBorderColor: $primary-content;
 
     .mx_SpacePanel_contextMenu_header {
         margin: 12px 16px 12px;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         font-size: $font-15px;
         line-height: $font-18px;
         overflow: hidden;
@@ -432,7 +432,7 @@ $activeBorderColor: $primary-content;
         color: $tertiary-content;
         font-size: $font-10px;
         line-height: $font-12px;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         //margin-left: 8px;
     }
 }