Particles. Intentionally omitted the blockcrack particle customization, i think it looks fine

src/main/java/io/github/cottonmc/templates/api/ThemeableBlockEntity.java

@@ -0,0 +1,7 @@
+package io.github.cottonmc.templates.api;
+
+import net.minecraft.block.BlockState;
+
+public interface ThemeableBlockEntity {
+	BlockState getThemeState();
+}

src/main/java/io/github/cottonmc/templates/block/TemplateBlock.java

@@ -5,7 +5,6 @@ import net.minecraft.block.Block;
 import net.minecraft.block.BlockEntityProvider;
 import net.minecraft.block.BlockState;
 import net.minecraft.block.Blocks;
-import net.minecraft.block.entity.BlockEntity;
 import net.minecraft.entity.LivingEntity;
 import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.item.BlockItem;
@@ -79,7 +78,7 @@ public abstract class TemplateBlock extends Block implements BlockEntityProvider
 		}
 		
 		//Changing the theme
-		if(held.getItem() instanceof BlockItem bi && be.getRenderedState().getBlock() == Blocks.AIR) {
+		if(held.getItem() instanceof BlockItem bi && be.getThemeState().getBlock() == Blocks.AIR) {
 			Block block = bi.getBlock();
 			ItemPlacementContext ctx = new ItemPlacementContext(new ItemUsageContext(player, hand, hit));
 			BlockState placementState = block.getPlacementState(ctx);
@@ -104,8 +103,10 @@ public abstract class TemplateBlock extends Block implements BlockEntityProvider
 		if(!state.isOf(newState.getBlock()) && world.getBlockEntity(pos) instanceof TemplateEntity template) {
 			DefaultedList<ItemStack> drops = DefaultedList.of();
 			
-			Block theme = template.getRenderedState().getBlock();
+			//TODO: remember the specific ItemStack
+			Block theme = template.getThemeState().getBlock();
 			if(theme != Blocks.AIR) drops.add(new ItemStack(theme));
+			
 			if(template.hasSpentRedstoneTorch()) drops.add(new ItemStack(Items.REDSTONE_TORCH));
 			if(template.hasSpentGlowstoneDust()) drops.add(new ItemStack(Items.GLOWSTONE_DUST));
 			

src/main/java/io/github/cottonmc/templates/block/entity/TemplateEntity.java

@@ -1,6 +1,7 @@
 package io.github.cottonmc.templates.block.entity;
 
 import io.github.cottonmc.templates.Templates;
+import io.github.cottonmc.templates.api.ThemeableBlockEntity;
 import net.fabricmc.fabric.api.rendering.data.v1.RenderAttachmentBlockEntity;
 import net.minecraft.block.BlockState;
 import net.minecraft.block.Blocks;
@@ -18,7 +19,7 @@ import org.jetbrains.annotations.Nullable;
 
 import java.util.Objects;
 
-public class TemplateEntity extends BlockEntity implements RenderAttachmentBlockEntity {
+public class TemplateEntity extends BlockEntity implements RenderAttachmentBlockEntity, ThemeableBlockEntity {
 	protected BlockState renderedState = Blocks.AIR.getDefaultState();
 	
 	//Whether the player has manually spent a redstone/glowstone item to upgrade the template.
@@ -74,7 +75,8 @@ public class TemplateEntity extends BlockEntity implements RenderAttachmentBlock
 		return renderedState;
 	}
 	
-	public BlockState getRenderedState() {
+	@Override
+	public BlockState getThemeState() {
 		return renderedState;
 	}
 	

src/main/java/io/github/cottonmc/templates/mixin/particles/MixinEntity.java

@@ -0,0 +1,33 @@
+package io.github.cottonmc.templates.mixin.particles;
+
+import io.github.cottonmc.templates.api.ThemeableBlockEntity;
+import net.minecraft.block.BlockState;
+import net.minecraft.block.Blocks;
+import net.minecraft.entity.Entity;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.World;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.ModifyArg;
+
+@Mixin(Entity.class)
+public abstract class MixinEntity {
+	@Shadow @Deprecated public abstract BlockPos getLandingPos(); //TODO, somewhat expensive method
+	
+	@ModifyArg(
+		method = "spawnSprintingParticles",
+		at = @At(value = "INVOKE", target = "Lnet/minecraft/particle/BlockStateParticleEffect;<init>(Lnet/minecraft/particle/ParticleType;Lnet/minecraft/block/BlockState;)V"),
+		require = 0
+	)
+	private BlockState templates$spawnSprintingParticles$modifyParticleState(BlockState origState) {
+		World world = ((Entity) (Object) this).getWorld();
+		
+		if(world.getBlockEntity(getLandingPos()) instanceof ThemeableBlockEntity themeable) {
+			BlockState theme = themeable.getThemeState();
+			if(theme.getBlock() != Blocks.AIR) return theme;
+		}
+		
+		return origState;
+	}
+}

src/main/java/io/github/cottonmc/templates/mixin/particles/MixinLivingEntity.java

@@ -0,0 +1,41 @@
+package io.github.cottonmc.templates.mixin.particles;
+
+import io.github.cottonmc.templates.api.ThemeableBlockEntity;
+import net.minecraft.block.BlockState;
+import net.minecraft.block.Blocks;
+import net.minecraft.entity.Entity;
+import net.minecraft.entity.LivingEntity;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.World;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Unique;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.ModifyArg;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(LivingEntity.class)
+public class MixinLivingEntity {
+	@Unique private BlockPos lastFallCheckPos;
+	
+	@Inject(method = "fall", at = @At("HEAD"))
+	private void templates$onFall(double d, boolean bl, BlockState blockState, BlockPos blockPos, CallbackInfo ci) {
+		lastFallCheckPos = blockPos;
+	}
+	
+	@ModifyArg(
+		method = "fall",
+		at = @At(value = "INVOKE", target = "Lnet/minecraft/particle/BlockStateParticleEffect;<init>(Lnet/minecraft/particle/ParticleType;Lnet/minecraft/block/BlockState;)V"),
+		require = 0
+	)
+	private BlockState templates$fall$modifyParticleState(BlockState origState) {
+		World world = ((Entity) (Object) this).getWorld();
+		
+		if(lastFallCheckPos != null && world.getBlockEntity(lastFallCheckPos) instanceof ThemeableBlockEntity themeable) {
+			BlockState theme = themeable.getThemeState();
+			if(theme.getBlock() != Blocks.AIR) return theme;
+		}
+		
+		return origState;
+	}
+}

src/main/resources/assets/templates/models/block/slope_base.json

@@ -1,6 +1,9 @@
 {
 	"parent": "block/block",
 	"gui_light": "front",
+	"textures": {
+		"particle": "block/scaffolding_top"
+	},
 	"display": {
 		"firstperson_righthand": {
 			"rotation": [ 0, 135, 0 ],

src/main/resources/fabric.mod.json

@@ -9,6 +9,9 @@
 	"contact": {
 		"sources": "https://github.com/CottonMC/Templates"
 	},
+	"mixins": [
+		"templates.mixins.json"
+	],
 	"environment": "*",
 	"entrypoints": {
 		"main": [
@@ -21,7 +24,6 @@
 	"depends": {
 		"minecraft": ">=1.20.1",
 		"fabricloader": "*",
-		"fabric-api": "*",
+		"fabric-api": "*"
-		"fabric-renderer-indigo": "*"
 	}
 }

src/main/resources/templates.mixins.json

@@ -0,0 +1,13 @@
+{
+	"required": true,
+	"package": "io.github.cottonmc.templates.mixin",
+	"compatibilityLevel": "JAVA_17",
+	"mixins": [
+		"particles.MixinEntity",
+		"particles.MixinLivingEntity"
+	],
+	"injectors": {
+		"defaultRequire": 1
+	},
+	"refmap": "packages.refmap.json"
+}