You can adjust the format of your chart’s axis text, one axis at a time. Click on an axis, and open the presentation properties tab at the top of the window. In the appearance group, you can change the text color and font.
Click on the “…” button next to font to select from more fonts and adjust the size.
If you want to re-use the selected font and color for the axis text of many charts, you can save it as part of a style sheet which you can easily apply to your charts.