Registration screen improvements (#115)

* Fix 'You must first log in' text alignment.

* Add bottom margin.

* Fix reading width, fix network footer primary colour.

assets/styles/common/_global.scss

@@ -1 +1,4 @@
 // TODO
+:root {
+  --reading-width: 50em;
+}

assets/styles/layouts/_footer.scss

@@ -1 +1,4 @@
 // TODO
+.footer__network {
+  --primary: --accent-fg;
+}

assets/styles/layouts/_signup.scss

@@ -1,8 +1,6 @@
 .mu_register.wp-signup-container {
    @extend .page-section;
-   margin: rem(120) auto 0;
-   display: flex;
-   flex-direction: column;
+   margin: rem(120) auto 4rem;
 
    * {
      text-align: left;

dist/mix-manifest.json

@@ -1,6 +1,6 @@
 {
     "/scripts/aldine.js": "/scripts/aldine.js?id=e222d7e1e1c71c6bdb98",
-    "/styles/aldine.css": "/styles/aldine.css?id=457f732469b0acd4ab09",
+    "/styles/aldine.css": "/styles/aldine.css?id=318adc125f771c02e1c4",
     "/styles/editor.css": "/styles/editor.css?id=e96b3d10ea63fa07d09b",
     "/scripts/customizer.js": "/scripts/customizer.js?id=0a52a1e0117b2902b145",
     "/scripts/catalog-admin.js": "/scripts/catalog-admin.js?id=4ee1f7f0640e28791cf2",