Replace button width sass variable with a css one

src/styles/_page.project.scss

@@ -13,6 +13,7 @@
 	--track-main: #{color.$main-track};
 	--track-off-track: #{color.$off-track};
 	--track-hitchhiking: #{color.$hitchhiking};
+	--button-width: 31px;
 
 	overflow: hidden;
 	position: absolute;