Is there a way of making the Active Tab Border highlighted on top instead of at the bottom?

You can visit the Theme Color VS Code web page to get more information on this.

Open your user settings.json (Ctrl + ,)

Two lines below the tab.activeBorder, you will find tab.activeBorderTop, which does exactly what you intended.

"workbench.colorCustomizations": {       // Can customize each aspect
    "[One Dark Pro]": {                  // Optional
        "tab.activeBorderTop": "#0A84FF" // Active Tab Top Highlighting
    }
},