In this guide, we provide examples of how Streamlit page elements are affected by the various theme config options. For a more high-level overview of Streamlit themes, see the Themes section of the main concepts documentation.

Streamlit themes are defined using regular config options: a theme can be set via command line flag when starting your app using streamlit run or by defining it in the [theme] section of a .streamlit/config.toml file. For more information on setting config options, please refer to the Streamlit configuration documentation.

The following config options show the default Streamlit Light theme recreated in the [theme] section of a .streamlit/config.toml file.

[theme]
primaryColor="#F63366"
backgroundColor="#FFFFFF"
secondaryBackgroundColor="#F0F2F6"
textColor="#262730"
font="sans serif"

Let's go through each of these options, providing screenshots to demonstrate what parts of a Streamlit app they affect where needed.

primaryColor defines the accent color most often used throughout a Streamlit app. A few examples of Streamlit widgets that use primaryColor include st.checkbox, st.slider, and st.text_input (when focused).

.. tip::
   Any CSS color can be used as the value for primaryColor and the other color
   options below. This means that theme colors can be specified in hex or with
   browser-supported color names like "green", "yellow", and
   "chartreuse". They can even be defined in the RGB and HSL formats!

Defines the background color used in the main content area of your app.

This color is used where a second background color is needed for added contrast. Most notably, it is the sidebar's background color. It is also used as the border and header color for st.dataframe, the plot background color for st.plotly_chart, and as the background color for most other interactive widgets.

This option controls the text color for most of your Streamlit app.

Selects the font used in your Streamlit app. Valid values are "sans serif", "serif", and "monospace". This option defaults to "sans serif" if unset or invalid.

Note that code blocks are always rendered using the monospace font regardless of the font selected here.

An easy way to define custom themes that make small changes to one of the preset Streamlit themes is to use the base option. Using base, the Streamlit Light theme can be recreated as a custom theme by writing the following:

[theme]
base="light"

The base option allows you to specify a preset Streamlit theme that your custom theme inherits from. Any theme config options not defined in your theme settings have their values set to those of the base theme. Valid values for base are "light" and "dark".

For example, the following theme config defines a custom theme nearly identical to the Streamlit Dark theme, but with a new primaryColor.

[theme]
base="dark"
primaryColor="purple"

If base itself is omitted, it defaults to "light", so you can define a custom theme that changes the font of the Streamlit Light theme to serif with the following config

[theme]
font="serif"