Set compose button color

app/styles/soapbox/components.scss

@@ -4,7 +4,7 @@
 }
 
 .button {
-  background-color: $ui-highlight-color;
+  background-color: var(--brand-color);
   border: 10px none;
   border-radius: 4px;
   box-sizing: border-box;
@@ -524,7 +524,7 @@
   padding-left: 68px;
   position: relative;
   min-height: 54px;
-  border-bottom: 1px solid lighten($ui-base-color, 8%);
+  border-bottom: 1px solid var(--brand-color-faint);
   cursor: default;
 
   @supports (-ms-overflow-style: -ms-autohiding-scrollbar) {