use variables for thumbnail size

src/renderer/scss/_gui.scss

@@ -303,7 +303,7 @@ p {
 
 .column {
   display: flex;
-  
+
   .column__item:not(:first-child) {
     padding-left: $spacing-width * 2/3;
   }
@@ -370,6 +370,6 @@ p {
 }
 
 .thumbnail-preview {
-  height: 100px;
-  width: 177px;
+  height: var(--thumbnail-preview-height);
+  width: var(--thumbnail-preview-width);
 }

src/renderer/scss/_vars.scss

@@ -179,4 +179,8 @@ $large-breakpoint: 1921px;
   // /* Animation :) */
   --animation-duration: 0.3s;
   --animation-style: cubic-bezier(0.55, 0, 0.1, 1);
+
+  /* Image */
+  --thumbnail-preview-height: 100px;
+  --thumbnail-preview-width: 177px;
 }

src/renderer/scss/component/_form-field.scss

@@ -68,7 +68,7 @@
   }
 
   input.input--thumbnail {
-    width: 370px;
+    width: 400px;
   }
 
   &.form-field--auto-height {