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Since version 3.0 Thonny supports UI themes, which allow changing the appearance of widgets, and Syntax themes, which allow specifying coloring scheme for editors and the shell. A couple of themes are built into Thonny (see "Tools => Options => Theme & Font", but users can add their own themes via the plug-in system.

Adding a syntax theme

Let's say you mostly like the built-in "Desert Sunset" theme, but you wish the keywords had lighter foreground and regular weight and current line was more pronounced. In this case it you can write a plug-in which adds a new syntax theme (let's call it "Desert Sunset Plus"), which is based on "Desert Sunset" but changes aforementioned properties.

A plug-in can add a new syntax theme via workbench method add_syntax_theme. You need to pass a unique name for your theme, name of the parent theme and a dictionary specifying visual properties for syntactical elements to be overridden. (Instead of the dictionary you can also provide a parameterless function returning the dictionary -- this way you can postpone computing the properties on theme loading time.)

Let's first inspect the original theme. All built-in syntax themes are defined in thonny.plugins.base_syntax_themes. Locate this file and find the line with get_workbench().add_syntax_theme("Desert Sunset", "Default Dark", desert_sunset). As we see its parent theme is "Default Dark" and it provides properties as a function. After investigating the properties of "Desert Sunset" and "Default Dark", you could come up with a plug-in like this:

The plug-in implementing "Desert Sunset Plus" could be something like this:

from thonny import get_workbench

def load_plugin():
    get_workbench().add_syntax_theme("Desert Sunset Plus", "Desert Sunset", {
        "keyword"       : {"foreground" : "#DE8C3A", "font" : "EditorFont"},
        "current_line"  : {"background" : "#525252"},

Look around in thonny.plugins.base_syntax_themes to learn about available syntactic and other elements and their configuration options.

Note that even if you want to create a whole new theme, it's recommended to specify a parent theme (either "Default Dark" or "Default Light") in order to benefit from fallback properties which may be introduced in future Thonny versions.

UI Themes

In order to create a new UI theme you need to first look around in thonny.plugins.base_ui_themes and thonny.plugins.clean_ui_themes.

In general you should be familiar with Tkinter (or Tk) ttk styles unless you just want to create a derivative of "Clean Dark" or "Clean Light" (in this case I'm sure you can figure it out from the examples of "Clean Dark Green" and friends).