#59, Allow customize SASS vars, add example of dark theme

exampleSite/assets/_custom.scss

@@ -1 +1,2 @@
 @import "variables";
+// You can add custom styles here.

exampleSite/assets/_variables.scss

@@ -0,0 +1,13 @@
+// You can override SASS variables here. Below example of simple dark theme.
+
+/*
+$gray-100: rgba(255, 255, 255, 0.1);
+$gray-200: rgba(255, 255, 255, 0.2);
+
+$body-background: #343a40;
+$body-font-color: #e9ecef;
+
+$color-link: #84b2ff;
+$color-visited-link: #b88dff;
+$color-dark-link: $body-font-color;
+*/

exampleSite/resources/_gen/assets/js/search-data.js_da973b2166501f614a67d55884e62fdb.json

@@ -1 +0,0 @@
-{"Target":"search-data.min.d8cb1397d75559602eb3c92473e1c14b41f6d6198ea34823cb48fc4298b46ee8.js","MediaType":"application/javascript","Data":{"Integrity":"sha256-2MsTl9dVWWAus8kkc+HBS0H21hmOo0gjy0j8Qpi0bug="}}

exampleSite/resources/_gen/assets/js/search.js_db9e329433ffca0798ed12c88043b3b3.content

@@ -1,5 +0,0 @@
-(function(){const input=document.querySelector("#book-search-input");const results=document.querySelector("#book-search-results");input.addEventListener("focus",init);input.addEventListener("keyup",search);function init(){input.removeEventListener("focus",init);input.required=true;loadScript("/theme/hugo-book/lunr.min.js");loadScript("/theme/hugo-book/search-data.min.d8cb1397d75559602eb3c92473e1c14b41f6d6198ea34823cb48fc4298b46ee8.js",function(){input.readOnly=false;input.required=false;search();});}
-function search(){while(results.firstChild){results.removeChild(results.firstChild);}
-if(!input.value){return;}
-const terms=lunr.tokenizer(input.value);const searchHits=window.bookSearch.idx.query(function(query){query.term(terms,{boost:100});query.term(terms,{boost:10,wildcard:lunr.Query.wildcard.LEADING|lunr.Query.wildcard.TRAILING});query.term(terms,{editDistance:2});});searchHits.slice(0,10).forEach(function(hit){const page=window.bookSearch.pages[hit.ref];const li=document.createElement("li"),a=li.appendChild(document.createElement("a"));a.href=page.href;a.textContent=page.title;results.appendChild(li);});}
-function loadScript(src,callback){const script=document.createElement("script");script.defer=true;script.src=src;script.onload=callback;document.head.append(script);}})();

exampleSite/resources/_gen/assets/js/search.js_db9e329433ffca0798ed12c88043b3b3.json

@@ -1 +0,0 @@
-{"Target":"search.min.da4874ea4f4a57ac6e394d9c0e5d4c8a0aee75879f3d801250bf2fe7d72e1ff9.js","MediaType":"application/javascript","Data":{"Integrity":"sha256-2kh06k9KV6xuOU2cDl1MigrudYefPYASUL8v59cuH/k="}}