Add borders between cells in tables

The base Spectre.css style for tables only has borders between rows, but
that can look pretty weird in larger tables. This is more of a normal
table styling.

tildes/scss/_base.scss

@@ -198,7 +198,7 @@ table {
 
 td,
 th {
-  border-bottom: $border-width solid;
+  border: $border-width solid;
   padding: $unit-3 $unit-2;
 }
 

tildes/scss/themes/_theme_base.scss

@@ -98,12 +98,13 @@
     background-color: map-get($theme, "background-secondary");
   }
 
-  td {
+  td,
+  th {
     border-color: map-get($theme, "border");
   }
 
   th {
-    border-color: map-get($theme, "foreground-highlight");
+    border-bottom-color: map-get($theme, "foreground-highlight");
   }
 
   .form-autocomplete {