Add documentation for customising JabRef using CSS, fixes JabRef#254

en/setup/cssCustomisation.md

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+# Customise the look of JabRef using CSS
+The look of JabRef can be customised by importing custom CSS files. Imported themes, as well as the two basic themes _Light_ and _Dark_, can be exported from JabRef in the form of CSS files.
+
+The custom CSS theme is applied on top of the standard _Light_ theme which means that your custom CSS file does not need to contain all the properties that JabRef needs, just add the once that you want to change.
+
+**If you want to know which CSS properties JabRef uses please export one of the base themes.**
+
+## Import CSS file
+To import a CSS file to JabRef click on Options -> Preferences, then on the _Import theme_ button in the lower-left corner. Now navigate to your CSS file using the opened dialogue box, select your file and click _open_.
+
+After the dialogue box has closed, go to the _Appearance_ tab (use the search box if you can't find it) in preferences and under _Visual theme_ chose the _Custom theme_ option. Then click _Save_ in the lower-right corner and restart the JabRef.
+
+Keep in mind that the changes will not take effect until you have restarted JabRef.
+
+## Export an existing theme
+To export a theme click on _Options_ -> _Preferences_,  then on the _Export theme_ button in the lower-left corner. Then click on the theme you want to export.