Example style-editor.css for applying custom style to Block Editor (Gutenberg) in WordPress 5.x+

style-editor.css

@font-face {
	font-family: "Inter var";
	font-weight: 100 900; /* stylelint-disable-line font-weight-notation */
	font-style: normal;
	font-display: swap;
	src: url('./assets/fonts/inter/Inter-upright-var.woff2') format("woff2");
}

@font-face {
	font-family: "Inter var";
	font-weight: 100 900; /* stylelint-disable-line font-weight-notation */
	font-style: italic;
	font-display: swap;
	src: url('./assets/fonts/inter/Inter-italic-var.woff2') format("woff2");
}

.editor-styles-wrapper {
	box-sizing: border-box;
	color: #000;
	font-family: -apple-system, BlinkMacSystemFont, "Helvetica Neue", Helvetica, sans-serif;
	font-size: 1rem;
	line-height: 1.4;
	text-align: left;
}

.editor-block-list__layout .editor-block-list__block,
.editor-block-list__layout .editor-block-list__block p,
.editor-post-title__block .editor-post-title__input,
.editor-block-list__layout .editor-block-list__block * {
	font-family: -apple-system, BlinkMacSystemFont, "Helvetica Neue", Helvetica, sans-serif;
}

@supports ( font-variation-settings: normal ) {
	.editor-styles-wrapper {
		font-family: "Inter var", -apple-system, BlinkMacSystemFont, "Helvetica Neue", Helvetica, sans-serif;
	}
}