feat: Allow semiliterate to adjust the theme

The mechanism here is to shadow a specified `custom.theme_dir` in the
  generated docs directory, and then ignore generated files therein (since they
  will be being used by the theme to render the pages, and are not actually
  part of the documentation).

tests/fixtures/theme-modification/refsite/css/theme_extra.css

@@ -0,0 +1,191 @@
+/*
+ * Wrap inline code samples otherwise they shoot of the side and
+ * can't be read at all.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/313
+ * https://github.com/mkdocs/mkdocs/issues/233
+ * https://github.com/mkdocs/mkdocs/issues/834
+ */
+.rst-content code {
+    white-space: pre-wrap;
+    word-wrap: break-word;
+    padding: 2px 5px;
+}
+
+/**
+ * Make code blocks display as blocks and give them the appropriate
+ * font size and padding.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/855
+ * https://github.com/mkdocs/mkdocs/issues/834
+ * https://github.com/mkdocs/mkdocs/issues/233
+ */
+.rst-content pre code {
+    white-space: pre;
+    word-wrap: normal;
+    display: block;
+    padding: 12px;
+    font-size: 12px;
+}
+
+/**
+ * Fix code colors
+ *
+ * https://github.com/mkdocs/mkdocs/issues/2027
+ */
+.rst-content code {
+    color: #E74C3C;
+}
+
+.rst-content pre code {
+    color: #000;
+    background: #f8f8f8;
+}
+
+/*
+ * Fix link colors when the link text is inline code.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/718
+ */
+a code {
+    color: #2980B9;
+}
+a:hover code {
+    color: #3091d1;
+}
+a:visited code {
+    color: #9B59B6;
+}
+
+/*
+ * The CSS classes from highlight.js seem to clash with the
+ * ReadTheDocs theme causing some code to be incorrectly made
+ * bold and italic.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/411
+ */
+pre .cs, pre .c {
+    font-weight: inherit;
+    font-style: inherit;
+}
+
+/*
+ * Fix some issues with the theme and non-highlighted code
+ * samples. Without and highlighting styles attached the
+ * formatting is broken.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/319
+ */
+.rst-content .no-highlight {
+    display: block;
+    padding: 0.5em;
+    color: #333;
+}
+
+
+/*
+ * Additions specific to the search functionality provided by MkDocs
+ */
+
+.search-results {
+    margin-top: 23px;
+}
+
+.search-results article {
+    border-top: 1px solid #E1E4E5;
+    padding-top: 24px;
+}
+
+.search-results article:first-child {
+    border-top: none;
+}
+
+form .search-query {
+    width: 100%;
+    border-radius: 50px;
+    padding: 6px 12px;  /* csslint allow: box-model */
+    border-color: #D1D4D5;
+}
+
+/*
+ * Improve inline code blocks within admonitions.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/656
+ */
+ .rst-content .admonition code {
+    color: #404040;
+    border: 1px solid #c7c9cb;
+    border: 1px solid rgba(0, 0, 0, 0.2);
+    background: #f8fbfd;
+    background: rgba(255, 255, 255, 0.7);
+}
+
+/*
+ * Account for wide tables which go off the side.
+ * Override borders to avoid weirdness on narrow tables.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/834
+ * https://github.com/mkdocs/mkdocs/pull/1034
+ */
+.rst-content .section .docutils {
+    width: 100%;
+    overflow: auto;
+    display: block;
+    border: none;
+}
+
+td, th {
+    border: 1px solid #e1e4e5 !important; /* csslint allow: important */
+    border-collapse: collapse;
+}
+
+/*
+ * Without the following amendments, the navigation in the theme will be
+ * slightly cut off. This is due to the fact that the .wy-nav-side has a
+ * padding-bottom of 2em, which must not necessarily align with the font-size of
+ * 90 % on the .rst-current-version container, combined with the padding of 12px
+ * above and below. These amendments fix this in two steps: First, make sure the
+ * .rst-current-version container has a fixed height of 40px, achieved using
+ * line-height, and then applying a padding-bottom of 40px to this container. In
+ * a second step, the items within that container are re-aligned using flexbox.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/2012
+ */
+ .wy-nav-side {
+    padding-bottom: 40px;
+}
+
+/*
+ * The second step of above amendment: Here we make sure the items are aligned
+ * correctly within the .rst-current-version container. Using flexbox, we
+ * achieve it in such a way that it will look like the following:
+ *
+ * [No repo_name]
+ *         Next >>                    // On the first page
+ * << Previous     Next >>            // On all subsequent pages
+ *
+ * [With repo_name]
+ *    <repo_name>        Next >>      // On the first page
+ * <repo_name>  << Previous  Next >>  // On all subsequent pages
+ *
+ * https://github.com/mkdocs/mkdocs/issues/2012
+ */
+.rst-versions .rst-current-version {
+    padding: 0 12px;
+    display: flex;
+    font-size: initial;
+    justify-content: space-between;
+    align-items: center;
+    line-height: 40px;
+}
+
+/*
+ * Please note that this amendment also involves removing certain inline-styles
+ * from the file ./mkdocs/themes/readthedocs/versions.html.
+ *
+ * https://github.com/mkdocs/mkdocs/issues/2012
+ */
+.rst-current-version span {
+    flex: 1;
+    text-align: center;
+}