Added example of custom css files (#6919)

* added example of custom css files

* prettify

Author: RRosio

docs/source/custom_css.md

@@ -0,0 +1,77 @@
+# Applying Custom CSS
+
+To apply custom CSS, you can add a `/custom/custom.css` file in the jupyter `config` directory. You can find the path, `~/.jupyter`, to this directory by running `jupyter --paths`. There you can create a folder named `custom` and create a `custom.css` file within the folder.
+
+## Jupyter Styling
+
+You can use a custom CSS file to modify default Jupyter styling.
+
+```
+/* Modify Jupyter Styles */
+#top-panel-wrapper, #jp-top-bar {
+  background-color: #aecad4!important;
+}
+
+#menu-panel-wrapper, #jp-MainMenu, #menu-panel {
+  background-color: #aecad4!important;
+}
+
+.jp-NotebookPanel-toolbar {
+  background-color:  #aecad4!important;
+}
+.lm-MenuBar-content {
+  color: #02484d
+}
+```
+
+![a screenshot custom jupyter styling](https://user-images.githubusercontent.com/12378147/245519958-17ce04e7-edc2-434e-8d93-a5c2de9fb225.png)
+
+## Markdown
+
+Another potential application for custom CSS is styling markdown.
+
+```
+/* Headings */
+h1,
+h2 {
+  font-family: Impact, Charcoal, sans-serif;
+  font-weight: bold;
+  text-shadow: 2px 2px 4px #000000;
+}
+
+h1 {
+  font-size: 52px;
+  margin-bottom: 40px;
+  color: #10929e;
+  text-decoration: underline;
+
+}
+
+h2 {
+  font-size: 448px;
+  margin-bottom: 32px;
+  color: #76b4be;
+  text-transform: uppercase;
+}
+
+/* Block Quotes */
+blockquote {
+  font-family: Georgia, serif;
+  font-size: 16px;
+  color: #19085c;
+  border-left: 8px solid #effffc;
+  background-color: #eafcff;
+  padding: 20px;
+}
+
+/* Lists */
+ul,
+ol {
+  font-family: Verdana, Geneva, sans-serif;
+  font-size: 18px;
+  color: #333333;
+  margin-bottom: 24px;
+}
+```
+
+![a screenshot of custom markdown styling](https://user-images.githubusercontent.com/12378147/245520291-968848d3-d336-4523-a046-023b15082ff8.png)

docs/source/user-documentation.md

@@ -9,6 +9,7 @@ notebook
 ui_components
 notebook_7_features
 examples/Notebook/examples_index.rst
+custom_css
 troubleshooting
 changelog
 ```