patch-diff-report-tool: should preserve stylesheet.css from jxr site

We do read sources generated by JXR.
unfortunately tool does not copy it to `diff` folder, so sources become plain text.

customization of css:
```

org.apache.maven.plugins
maven-jxr-plugin
${jxr-plugin.version}


${project.basedir}/jxr.css

**/.ci-temp/**/*



```

custom css, should use default + our customization for higlihting of target (anchored) line.
```
/* content of https://github.com/apache/maven-jxr/blob/master/maven-jxr-plugin/src/main/resources/stylesheet.css -- BEGIN */

body {
background-color: #fff;
font-family: Arial, Helvetica, sans-serif;
}

a:link {
color: #00f;
}
a:visited {
color: #00a;
}

a:active, a:hover {
color: #f30 !important;
}

ul, li {
list-style-type:none;
margin:0;
padding:0;
}

table td {
padding: 3px;
border: 1px solid #000;
}
table {
width:100%;
border: 1px solid #000;
border-collapse: collapse;
}

div.overview {
background-color:#ddd;
padding: 4px 4px 4px 0;
}
div.overview li, div.framenoframe li {
display: inline;
}
div.framenoframe {
text-align: center;
font-size: x-small;
}
div.framenoframe li {
margin: 0 3px 0 3px;
}
div.overview li {
margin:3px 3px 0 3px;
padding: 4px;
}
li.selected {
background-color:#888;
color: #fff;
font-weight: bold;
}

table.summary {
margin-bottom: 20px;
}
table.summary td, table.summary th {
font-weight: bold;
text-align: left;
padding: 3px;
}
table.summary th {
background-color:#036;
color: #fff;
}
table.summary td {
background-color:#eee;
border: 1px solid black;
}

em {
color: #A00;
}
em.comment {
color: #390;
}
.string {
color: #009;
}

#overview {
padding:2px;
}

hr {
height: 1px;
color: #000;
}

/* JXR style sheet */
.jxr_comment
{
color: #390;
}

.jxr_javadoccomment
{
color: #A00;
}

.jxr_string
{
color: #009;
}

.jxr_keyword
{
color: #000;
}
/* content of https://github.com/apache/maven-jxr/blob/master/maven-jxr-plugin/src/main/resources/stylesheet.css -- END */
/* BELOW ARE OUR CUSTOMIZATION */
:target {
background-color: #b2b2e1;
}

```

to make it look like: