Pele lets you generate Mermaid diagrams as themeable SVG

The relationship between tools and their visual output has always shaped how we think. When a tool locks you into a fixed aesthetic, it doesn't just constrain appearance; it constrains how you can communicate ideas to different audiences. Understanding diagramming tools, theme systems, and the layer between data and presentation reveals something important about how modern software balances convenience with control. What diagramming languages actually are Diagramming languages like Mermaid exist in a specific gap in software: they let non-designers write diagrams by typing structured text instead of clicking and dragging. A programmer or analyst describes relationships, flows, or structures in a simple syntax, and the tool renders it as a visual. This is fundamentally different from traditional design tools where you place each element by hand. The tradeoff has always been speed and reproducibility in exchange for less visual flexibility. Mermaid emerged as an open source project precisely because this need was widespread. People working in markdown files, code repositories, and documentation platforms wanted to embed diagrams without leaving their text editor. The tool reads a text description and outputs a diagram, usually as SVG (scalable vector graphics), which can be embedded in web pages, documents, and applications. This democratized diagram creation for people without design training. The constraint of locked aesthetics For years, when tools like Mermaid generated diagrams, the output came with built in colors, fonts, spacing, and line styles. You got a diagram, but you got it in the tool's visual language. This created a real problem in practice. A tech team might produce dozens of diagrams that looked distinctly "Mermaid made," clashing with brand guidelines or visual systems. A company wanting consistency across all its diagrams faced friction: either accept the default look or convert everything to a design tool and lose the convenience of text based generation. The themability question matters because it sits at the intersection of automation and customization. Most tools force a choice: be fast or be flexible. True themability means you don't have to choose. How SVG makes styling possible SVG is fundamentally different from raster image formats like PNG or JPEG. It's a vector format defined in code, meaning every shape, line, and text element can be described and modified. When a diagramming tool outputs SVG instead of a static image, it's outputting structured, semantic data about what each element is and where it sits. That structure creates an opportunity: CSS and styling rules can target specific elements. This is why SVG as output matters philosophically. It transforms a diagram from a finished product into a component you can work with. You can write style rules that say "make all flowchart decision nodes this color" or "apply this font family to all text labels." The diagram data and the