Update styles.css

[catalinmitrofan]

Updating the unicode character for the collapsed tree node, as the 25BA should be used. This unicode character displays the same symbol in mobile safari as well as on desktop browsers. The prior unicode character was displayed on mobile safari like in the attached picture.

[codecov]

Codecov Report

Merging #328 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #328   +/-   ##
=======================================
  Coverage   92.13%   92.13%           
=======================================
  Files          25       25           
  Lines        1030     1030           
  Branches      146      146           
=======================================
  Hits          949      949           
  Misses         53       53           
  Partials       28       28

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