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/_HomePage.pcss

@@ -37,7 +37,7 @@ limitations under the License.
     }
 
     h1 {
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         font-size: $font-32px;
         line-height: $font-44px;
         margin-bottom: 4px;
@@ -45,7 +45,7 @@ limitations under the License.
 
     h2 {
         margin-top: 4px;
-        font-weight: $font-semi-bold;
+        font-weight: var(--font-semi-bold);
         font-size: $font-18px;
         line-height: $font-25px;
         color: $muted-fg-color;
@@ -73,7 +73,7 @@ limitations under the License.
             word-break: break-word;
             box-sizing: border-box;
 
-            font-weight: 600;
+            font-weight: var(--font-semi-bold);
             font-size: $font-15px;
             line-height: $font-20px;
             color: #fff; /* on all themes */