Remove all style customisations for documentation, except banner. Fix #7987 (#7990)

All of those customisations predate #7956, which fixed the loading of the CSS, so they were likely only intended for our previous Sphinx theme.

docs/_static/css/custom.css

@@ -1,55 +1,3 @@
-body.wy-body-for-nav {
-    font-family: -apple-system, BlinkMacSystemFont, 'Helvetica Neue', Helvetica, Arial, sans-serif;
-    line-height: 1.5em;
-    color: #333; /*$color-grey-1*/
-    background-color: #e6e6e6; /*$color-grey-4*/
-}
-
-nav.wy-nav-side {
-    background-color: #333;  /*color-grey-1*/
-}
-
-.body--autocomplete-open .wy-nav-side {
-    position: absolute;
-    overflow: visible;
-}
-
-.body--autocomplete-open .wy-side-scroll {
-    overflow: visible;
-}
-
-div.wy-side-nav-search {
-    background-color: #007d7e; /*color-teal*/
-}
-
-.wy-nav-top {
-    background-color: #007d7e; /*color-teal*/
-}
-
-.wy-side-nav-search>a:hover, .wy-side-nav-search .wy-dropdown>a:hover {
-    background: None; /*background for logo*/
-}
-
-.wy-side-nav-search>div.version {
-    color: white;
-}
-
-.wy-side-nav-search input[type=text] {
-    border-color: #d9d9d9; /*color-grey-3*/
-}
-
-.logo {
-    transition: all 0.25s cubic-bezier(0.28, 0.15, 0, 2.1);
-}
-
-.logo:hover {
-    transform: rotate(4deg);
-}
-
-details {
-    margin-bottom: 1em;
-}
-
 /* Wagtail Space */
 .wagtailspace {
     background: #00676a;