Fix field lists after switch to HTML5 writer

- classic: Put the pink'ish colour back on the field type part (dt).
- basic: remove top margin on the field data to align with the type.
- basic: use grid display instead of flexbox to avoid fixed width on
  the field type part.

Other themes affected/fixed due to changes in basic:
- bizstyle (only technically, it already was fixed in
  sphinx-doc#6262)
- nature
- sphinxdoc

Fixes sphinx-doc#6604

sphinx/themes/basic/static/basic.css_t

@@ -520,23 +520,24 @@ dl.citation > dd:after {
 }
 
 dl.field-list {
-    display: flex;
-    flex-wrap: wrap;
+    display: grid;
+    grid-template-columns: fit-content(30%) auto;
 }
 
 dl.field-list > dt {
-    flex-basis: 20%;
     font-weight: bold;
     word-break: break-word;
+    padding-left: 0.5em;
+    padding-right: 5px;
 }
 
 dl.field-list > dt:after {
     content: ":";
 }
 
 dl.field-list > dd {
-    flex-basis: 70%;
-    padding-left: 1em;
+    padding-left: 0.5em;
+    margin-top: 0em;
     margin-left: 0em;
     margin-bottom: 0em;
 }

sphinx/themes/bizstyle/static/bizstyle.css_t

@@ -412,15 +412,10 @@ p.versionchanged span.versionmodified {
 
 dl.field-list > dt {
     color: white;
-    padding-left: 0.5em;
-    padding-right: 5px;
     background-color: #82A0BE;
 }
 
 dl.field-list > dd {
-    padding-left: 0.5em;
-    margin-top: 0em;
-    margin-left: 0em;
     background-color: #f7f7f7;
 }
 

sphinx/themes/classic/static/classic.css_t

@@ -287,7 +287,7 @@ code {
     font-size: 0.95em;
 }
 
-th {
+th, dl.field-list > dt {
     background-color: #ede;
 }