https://www.octotree.io (free version) dark style for GitHub dark mode. (Firefox)

README.md

before	after

how to use

relying on userContent.css

Go to https://github.com/settings/appearance and select Dark or Default to system.

1. go to about:support page
2. open the Profile Directory folder
3. create the chrome folder
4. create the userContent.css file, and paste userContent---Dark.css or userContent---Default_to_system.css
5. restart your browser

userContent---Dark.css

/*!
 * octotree dark mode for github dark mode.
 * @version 2020-12-10
 * @link https://gist.github.com/micalevisk/3b478b06f7d277b7d274fab6889cb7b5
 * @author Micael Levi https://github.com/micalevisk
 * @license https://creativecommons.org/licenses/by-sa/4.0
 */
@-moz-document domain(github.com) {
  /**/
  .octotree-toggle-icon::before {
    color: var(--color-topic-tag-text) !important;
  }
  /**/
  .octotree-toggle>span {
    color: var(--color-topic-tag-text) !important;
  }
  /**/
  .octotree-toggle {
    background: var(--color-bg-canvas) !important;
    border: 1px solid var(--color-border-primary) !important;
  }
  /* tree view background */
  .octotree-views {
    background-color: var(--color-bg-canvas) !important;
  }
  /**/
  .octotree-sidebar.octotree-github-sidebar .octotree-views {
    border-right: 1px solid var(--color-border-primary) !important;
  }
  /**/
  .octotree-view-header {
    background-color: var(--color-header-bg) !important;
  }
  /**/
  .octotree-footer {
    background-color: var(--color-bg-canvas) !important;
    border-color: var(--color-border-primary) !important;
  }
  /**/
  .octotree-footer-user-profile__email a {
    color: var(--color-text-secondary) !important;
  }
  /* files name */
  .octotree-sidebar.octotree-github-sidebar .octotree-views .octotree-tree-view .jstree-anchor {
    color: var(--color-text-primary) !important;
  }
  /**/
  .jstree-wholerow-clicked {
    background: var(--color-bg-tertiary) !important;
  }
  /**/
  .jstree-open>.jstree-ocl::before {
    color: var(--color-topic-tag-text) !important;
  }
  /**/
  .jstree-closed>.jstree-ocl::before {
    color: var(--color-text-primary) !important;
  }
  /* (bugfix: when open the sidebar, the icon of the current file has gone) */
  .jstree-node[aria-selected="true"].jstree-leaf .octotree-default-icon.blob::before {
    color: #6a737d !important;
  }
  /**/
  .octotree-icon-file-directory::before {
    color: var(--color-files-explorer-icon) !important;
  }
  /**/
  .jstree-wholerow-hovered,
  .jstree-hovered {
    background: var(--color-bg-tertiary) !important;
  }
  /**/
  .jstree-default>.jstree-wholerow-ul .jstree-clicked,
  .jstree-default>.jstree-wholerow-ul .jstree-hovered {
    background: 0 0 !important;
  }
}

userContent---Default_to_system.css

/*!
 * octotree dark mode for github dark mode.
 * @version 2020-12-10
 * @link https://gist.github.com/micalevisk/3b478b06f7d277b7d274fab6889cb7b5
 * @author Micael Levi https://github.com/micalevisk
 * @license https://creativecommons.org/licenses/by-sa/4.0
 */
@-moz-document domain(github.com) {
  @media (prefers-color-scheme: dark) {
    /**/
    .octotree-toggle-icon::before {
      color: var(--color-topic-tag-text) !important;
    }
    /**/
    .octotree-toggle>span {
      color: var(--color-topic-tag-text) !important;
    }
    /**/
    .octotree-toggle {
      background: var(--color-bg-canvas) !important;
      border: 1px solid var(--color-border-primary) !important;
    }
    /* tree view background */
    .octotree-views {
      background-color: var(--color-bg-canvas) !important;
    }
    /**/
    .octotree-sidebar.octotree-github-sidebar .octotree-views {
      border-right: 1px solid var(--color-border-primary) !important;
    }
    /**/
    .octotree-view-header {
      background-color: var(--color-header-bg) !important;
    }
    /**/
    .octotree-footer {
      background-color: var(--color-bg-canvas) !important;
      border-color: var(--color-border-primary) !important;
    }
    /**/
    .octotree-footer-user-profile__email a {
      color: var(--color-text-secondary) !important;
    }
    /* files name */
    .octotree-sidebar.octotree-github-sidebar .octotree-views .octotree-tree-view .jstree-anchor {
      color: var(--color-text-primary) !important;
    }
    /**/
    .jstree-wholerow-clicked {
      background: var(--color-bg-tertiary) !important;
    }
    /**/    
    .jstree-open > .jstree-ocl::before {
      color: var(--color-topic-tag-text) !important;
    }
    /**/
    .jstree-closed > .jstree-ocl::before {
	    color: var(--color-text-primary) !important;
    }
    /* (bugfix: when open the sidebar, the icon of the current file has gone) */
    .jstree-node[aria-selected="true"].jstree-leaf .octotree-default-icon.blob::before {
	    color: #6a737d !important;
    }
    /**/
    .octotree-icon-file-directory::before {
      color: var(--color-files-explorer-icon) !important;
    }
    /**/
    .jstree-wholerow-hovered,
    .jstree-hovered {
      background: var(--color-bg-tertiary) !important;
    }
    /**/
    .jstree-default>.jstree-wholerow-ul .jstree-clicked,
    .jstree-default>.jstree-wholerow-ul .jstree-hovered {
      background: 0 0 !important;
    }
  }
}

and if you want to remove the "Octotree" label of the sidebar toggle button:

    .octotree-toggle__brand {
      display: none !important;
    }