docs: update testing guidance, version requirement, and button color fix across all languages

docs/images/README.md

@@ -1,6 +1,6 @@
 # Style Preview Images
 
-Place 600px-wide PNG screenshots here, captured from the [live preview](../preview/index.html).
+Place theme style screenshots of **Desktop** in PNG format here, captured from the [live preview](../preview/index.html).
 
 ## Naming Convention
 
@@ -11,14 +11,16 @@ neon.png       mono.png        terra.png    ink.png      aurora.png
 
 ## Image Size Requirements
 
+To ensure screenshots can be displayed in the README, please follow these size requirements:
+
 - **Maximum:** 50 KiB per image
-- **Recommended:** 10–20 KiB
+- **Recommended:** 30–40 KiB
 - **Format:** PNG, optimised — run through `pngquant` or `optipng` before committing
 
 ## How to Capture
 
-1. Start the dev server: `cd tools && go run . dev` (opens http://localhost:3456)
-2. For each style, select the "Activate Account" template and "Modern" client mode
+1. Start the dev server: `cd tools && go run . dev` (opens http://localhost:3456 in your browser)
+2. For each style, select the **"Activate Account"** template and **"Modern"** client mode in **"Desktop"**
 3. Take a screenshot of the rendered email (600px width recommended)
 4. Save as `<style-name>.png` in this directory