[Raed667]

Could it be that you're using Refined Github? Try disabling it.

[Seirdy]

A quick 'n dirty userstyle for Refined GH users:

    @-moz-document domain("github.com") {
     .dashboard .js-all-activity-header + div {
      background-color: inherit !important;
      border: 1px solid var(--color-border-secondary) !important;
     }
     .dashboard-rollup-items .body {
      border-top: 1px solid var(--color-border-secondary) !important;
     }
    }

Edit: issue for Refined GH: https://github.com/sindresorhus/refined-github/issues/3798

[Seirdy]

Refined GitHub was just updated to support it a few mins ago. A couple rough edges but it works now.