Minor CSS fixes for the new room list (#29334)

* refactor: use `var(--cpd-border-width-1)` instead of `1px`

* refactor: use header font in header

Author: florianduros

playwright/snapshots/left-panel/room-list-view/room-list-header.spec.ts/room-list-header-linux.png

@@ -11,7 +11,7 @@
 
     h1 {
         all: unset;
-        font: var(--cpd-font-body-lg-semibold);
+        font: var(--cpd-font-heading-sm-semibold);
     }
 
     button {

playwright/snapshots/left-panel/room-list-view/room-list-view.spec.ts/room-list-view-linux.png

@@ -9,7 +9,7 @@
     /* From figma, this should be aligned with the room header */
     height: 64px;
     box-sizing: border-box;
-    border-bottom: 1px solid var(--cpd-color-bg-subtle-primary);
+    border-bottom: var(--cpd-border-width-1) solid var(--cpd-color-bg-subtle-primary);
     padding: 0 var(--cpd-space-3x);
 
     svg {