Chart.js/docs/general/fonts.md

Fonts

There are 4 special global settings that can change all of the fonts on the chart. These options are in Chart.defaults.global. The global font settings only apply when more specific options are not included in the config.

For example, in this chart the text will all be red except for the labels in the legend.

Chart.defaults.global.defaultFontColor = 'red';
let chart = new Chart(ctx, {
    type: 'line',
    data: data,
    options: {
        legend: {
            labels: {
                // This more specific font property overrides the global property
                fontColor: 'black'
            }
        }
    }
});

Name	Type	Default	Description
defaultFontColor	Color	'#666'	Default font color for all text.
defaultFontFamily	String	"'Helvetica Neue', 'Helvetica', 'Arial', sans-serif"	Default font family for all text.
defaultFontSize	Number	12	Default font size (in px) for text. Does not apply to radialLinear scale point labels.
defaultFontStyle	String	'normal'	Default font style. Does not apply to tooltip title or footer. Does not apply to chart title.