Theme.json example showing how you can set up different simple presets for various blocks.

Theme.json example for simple presets

{
	"schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"settings": {
	      "layout": {
              		"contentSize": "800px",
              		"wideSize": "1000px"
        },
	      "color": {
              		"custom": true,
              		"customGradient": true,
              		"customDuotone": true,
              		"gradients": [],
              		"link": true,
              		"palette": [
                {
                    		"name": "Primary",
                    		"slug": "blue",
                    		"color": "#0288d1"
		              
                },
                {
                    		"name": "Secondary",
                    		"slug": "orange",
                    		"color": "#ff5837"
		                
                },
                {
                    		"name": "Tertiary",
                    		"slug": "grey",
                    		"color": "#282828"
		  
                }
          ]
        },
		"typography": {
			"customFontSize": true,
			"customLineHeight": true,
			"customFontWeight": true,
			"dropCap": true,
			"fontFamilies": [
            		{
         			  		"fontFamily": "Helvetica Neue, Helvetica, Arial, sans-serif",
          			   		"slug": "helvetica-arial"
        			},
       			 	{
          					"fontFamily": "Geneva, Tahoma, Verdana, sans-serif",
          					"slug": "geneva-verdana"
					}
      			]
   			 },
		"blocks": {
			"core/cover": {
				"color": {
					"duotone": [
                			  	{
                   		 		"colors": [ "#f0c443", "#ab0400" ],
                    	 		        "slug": "yellow-and-red",
                    	            		"name": "Yellow and Red"
               					  }
								   ]
	  						
  }
  },
			"core/image": {
				"color": {
                          		"duotone": [],
                          		"customDuotone": false
  }
  },

              		"core/post-featured-image": {
                   		"color": {
                          		"duotone": [
							  {
                   		 		        "colors": [ "#282828", "#ff5837" ],
                   		 		        "slug": "black-and-orange",
                   		 		        "name": "Black and Orange"
               					 },
							  {
                   		 		        "colors": [ "#282828", "#0288d1" ],
                   		 		        "slug": "black-and-blue",
                   		 		        "name": "Black and Blue"
               					 }
						  ],
                          		"customDuotone": true,
                          		"custom": false
	}
	  						
  },
              		"core/post-title": {
                    		"color": {
                         		"custom": false,
                          		"customGradient": false,
                          		"palette": []
          },
                    		"typography" : {
                          		"customFontSize": false,
                          		"customFontWeight": false,
                          		"customLineHeight": false,
                          		"fontSizes": [
                           { 			
                   		 		        "slug": "x-small",
                   		 		        "size": 25,
                   		 		        "name": "Small"
                       			
                            },
                     {
                      			
                   		 		        "slug": "x-large",
                   		 		        "size": 35,
                   		 		        "name": "Huge"
					 }
					]
          }
        },
        
              		"core/post-date": {
                    		"color": {
                          		"custom": false,
                          		"customGradient": false,
                          		"palette": []
          },
                    		"typography" : {
                          		"customFontSize": false,
                          		"customFontWeight": false,
                          		"customLineHeight": false,
                          		"fontSizes": [
                     { 			
                   		 		        "slug": "x-small",
                   		 		        "size": 15,
                   		 		        "name": "Small"
                       			
                      },
                     {
                      			
                   		 		        "slug": "x-large",
                   		 		        "size": 20,
                   		 		        "name": "Huge"
					 }
					]
          }
        }
  }

  }
		}