Fix dark mode for points in Charts
Summary:
* Change point color from `fill: #ffffff; stroke: {$blue}` to `fill: {$lightgreybackground}; stroke: {$darkgreyborder}` to allow replacing these variables in dark mode for improved readability
* Also reduce point and line stroke-width from 2px to 1px for improved readability (when lines are very close or overlap)
Refs T15056
Test Plan:
* Enable the "Facts" application at http://phorge.localhost/applications/view/PhabricatorFactApplication/
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to http://phorge.localhost/project/reports/1/ and look at the lines and dots in the chart
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26234