Various CSS improvements (#514)

* Replace flexbox grid with CSS grid

* Update new and edit forms

* Update settings views

* Update auth views

* Fix margin in menu

* Remove unused Spectre modules

* Simplify navbar

* Reuse CSS variables

* Fix grid gap on small screen sizes

* Simplify grid system

* Improve section headers

* Restructure SASS files

* Cleanup base styles

* Update test

bookmarks/styles/theme-dark.scss

@@ -2,16 +2,49 @@
 @import "variables-dark";
 
 // Import Spectre CSS lib
-@import "../../node_modules/spectre.css/src/spectre";
-@import "../../node_modules/spectre.css/src/autocomplete";
+@import "spectre";
 
 // Import style modules
 @import "base";
-@import "util";
-@import "shared";
-@import "bookmarks";
+@import "responsive";
+@import "bookmark-page";
+@import "bookmark-form";
 @import "settings";
-@import "auth";
 
-// Dark theme overrides
-@import "dark";
+/* Dark theme overrides */
+
+// Buttons
+.btn.btn-primary {
+  background: $dt-primary-button-color;
+  border-color: darken($dt-primary-button-color, 5%);
+
+  &:hover, &:active, &:focus {
+    background: darken($dt-primary-button-color, 5%);
+    border-color: darken($dt-primary-button-color, 10%);
+  }
+}
+
+// Focus ring
+a:focus, .btn:focus {
+  box-shadow: 0 0 0 .1rem rgba($primary-color, .5);
+}
+
+// Forms
+.form-input:not(:placeholder-shown):invalid,
+.form-input:not(:placeholder-shown):invalid:focus,
+.has-error .form-input,
+.form-input.is-error,
+.has-error .form-select,
+.form-select.is-error {
+  background: darken($error-color, 40%);
+}
+
+.form-checkbox input:checked + .form-icon, .form-radio input:checked + .form-icon, .form-switch input:checked + .form-icon {
+  background: $dt-primary-button-color;
+  border-color: $dt-primary-button-color;
+}
+
+// Pagination
+.pagination .page-item.active a {
+  background: $dt-primary-button-color;
+}