Dark square (besides arrow) keeps on text on non-widescreens (

The text of a site needs to be not on very rigth side — otherwise the dark square with an arrow disturbs reading, and also it does not work when on the text area.
As with other plugins with such a type — first loading of homepage increases about 15-20 % (J-query needs to be downloaded; total amount 38 kB more css. Yet I have found no plugin for such a job without activating j-query/j-query migrate:-(