It's common to want to re-create and style in your own brand a graph with information you've found in an article or online. When you find that information, it's often in a flat, uneditable image file, so you cannot update the colors, the legends, or the thickness of the lines.
However, PowerPoint has embedded tools to help you trace out a line graph and edit it for your own needs.
How to recreate a graph in an image I found online
Paste the graph image into your slide in the final position you want the graph to appear
Use the Insert --> Shapes --> Freeform Shape tool to trace out the lines of the graph
Delete the background graph
Edit the line thickness and colors to fit your brand
Recreate the legend, axes or any other elements you wish
Feel free to customize further by adding custom line markers (see video below) to bring some unexpectedness and delight to your audience.