add explicit height and remove card-margin variable

src/renderer/scss/_gui.scss

@@ -160,6 +160,7 @@ p {
     'nav header'
     'nav content';
   background-color: var(--color-bg);
+  height: 100vh;
 
   @media only screen and (min-width: $medium-breakpoint) {
     grid-template-columns: var(--side-nav-width-m) auto;

src/renderer/scss/_vars.scss

@@ -139,7 +139,6 @@ $large-breakpoint: 1760px;
   --card-bg: var(--color-white);
   --card-text-color: var(--color-grey-dark);
   --card-radius: 2px;
-  --card-margin: $spacing-vertical * 2/3;
   --card-wallet-color: var(--text-color-inverse);
   --success-msg-color: var(--color-green);
   --success-msg-border: var(--color-green-blue);

src/renderer/scss/component/_card.scss

@@ -177,7 +177,7 @@
 }
 
 .card__content {
-  margin-top: var(--card-margin);
+  margin-top: $spacing-vertical * 2/3;
 }
 
 .card__subtext-title {
@@ -198,7 +198,7 @@
 }
 
 .card__actions {
-  margin-top: var(--card-margin);
+  margin-top: $spacing-vertical * 2/3;
   display: flex;
 
   &:not(.card__actions--vertical) .btn:nth-child(n + 2) {