Without further a due, let's get started since I'm rushing right now.
1. Dashboard -> Design -> Edit HTML -> Tick Expand Widget
2. Ctrl+F the code /* Header
3. Copy the code below and paste it above the previous code.
border-bottom: 2px double #F52887;
- Red : Change it with how thick you want the line to be.
- Green : You can replace with DOTTED, NONE, SOLID or DASHED.
- Purple : That's the colour code. Can look it up from here.