docs: create CustomThemes.md (#1210)

* Create CustomThemes.md

* Update doc/CustomThemes.md
Co-authored-by: boojack <stevenlgtm@gmail.com>

* Update doc/CustomThemes.md
Co-authored-by: boojack <stevenlgtm@gmail.com>

* Update doc/CustomThemes.md
Co-authored-by: boojack <stevenlgtm@gmail.com>

* Update CustomThemes.md

* Update CustomThemes.md

---------
Co-authored-by: boojack <stevenlgtm@gmail.com>

doc/CustomThemes.md

+# Adding A Custom Theme
+
+1. Open the Settings Dialog
+2. Navigate to the System Tab
+3. In the "Additional Styles" box add these lines of code:
+
+```css
+.memo-list-container {background-color: #INSERT COLOR HERE;}
+.page-container {background-color: #INSERT COLOR HERE;}
+```
+
+  It is recommended that you choose the same color for both options
+
+4. Refresh the page and the background color of your memos app will successfully update to reflect your changes