Is it possible to have the left sidebar open by default when first loading the page?

To resolve this issue, you can do as the following.
– Open the sidebar.php file in the folder path “wp-content\themes\dw-timeline\templates” Find the line 31
Replace the following code:

  <aside class="sidebar sidebar-primary" role="complementary" style="position: absolute; display: none;">

With new code:
 

<aside class="sidebar sidebar-primary" role="complementary" style="position: fixed; display: block; overflow-y: scroll;">

– Open the  custom.php file in the folder path “wp-content\themes\dw-timeline\lib”. Find the line 323 to line 333.
Replace the following code:

function dw_timeline_html_class() {
  $classes = array();
  if ( is_404() ) {
    $classes[] = 'error404-html';
  } else if ( is_single() ) {
    $classes[] = 'single-html';
  }
  foreach ($classes as $key => $class) {
    echo $class;
  }
}

With new code:

function dw_timeline_html_class() {
  $classes = array();
  if ( is_404() ) {
    $classes[] = 'error404-html';
  } else if ( is_single() ) {
    $classes[] = 'single-html';
  }

  $classes[] .= 'main-sidebar-open';
  
  foreach ($classes as $key => $class) {
    echo $class;
  }
}

Hope this helps !