💄 Use of border-box box sizing

This makes calculating the size of the UI components easier, especially those which have padding, margin or border.

src/ui/_base.scss

@@ -194,9 +194,18 @@ $transition-duration: 300ms;
     font-weight: bold;
 }
 
+// Cf. https://css-tricks.com/box-sizing/
+html {
+  box-sizing: border-box;
+}
+*, *:before, *:after {
+  box-sizing: inherit;
+}
+
 body {
     height: 100vh;
     width: 100vw;
+
     margin: 0px;
     padding: 0px;
     outline: 0px;