Here's how to customize chart backgrounds and gridlines in Excel:

Chart Background:

1. Select the chart: Click anywhere on the chart to activate it.
2. Format Chart Area: On the "Chart Tools" ribbon tab (which appears when you select the chart), locate the "Format" section. Click on "Format Chart Area."
3. Fill Options: A formatting pane will appear on the right side. In the "Fill" section, you can choose various options for the chart background:
- Solid fill: Choose a solid color from the provided palette or click "Fill Effects" for more options like gradients, textures, or pictures.
- No fill: This removes any background color, making the chart transparent.
- Picture fill: You can browse and insert an image as your chart background.

Gridlines:

1. Select the chart: Click anywhere on the chart to activate it.
2. Chart Layouts: On the "Chart Tools" ribbon tab, navigate to the "Chart Layouts" group.
3. Gridlines options: Click the arrow next to "Gridlines." Here you'll see options for:
- Major gridlines: These are the main horizontal and vertical lines that divide the chart area. You can choose to show or hide them for both axes (horizontal and vertical).
- Minor gridlines: These are smaller lines that subdivide the major gridlines, providing more granularity. You can control their display for each axis as well.
4. Format Gridlines (Optional): Once you've chosen which gridlines to display, you can further customize them:
- Right-click on any gridline and select "Format Gridline."
- In the formatting pane, you can adjust the line color, style (solid, dashed, etc.), and weight (thickness).

Additional Tips:

- You can experiment with different background colors and gridline styles to find what best suits your data visualization needs.
- Consider the clarity and readability of your chart when making these customizations.
- Remember that chart formatting options might vary slightly depending on the specific chart type you're using.