Radzen Blazor Studio copies the CSS and fonts to your app when you use a premium theme such as Fluent. Other free themes are bundled in the Radzen.Blazor nuget package.
To override CSS you just specify the rules you want to override after the <link> which includes the Radzen Blazor theme e.g.:
<link rel="stylesheet" href="_content/Radzen.Blazor/css/software-base.css" />
<style>
.rz-textbox {
font-size: 20px !important; /* change the font size of all RadzenTextBox components */
}
</style>
The Radzen.Blazor themes heavily use CSS variables which makes overriding a lot easier and more flexible. For example to set the font-size of all textboxes you can also use:
.rz-textbox {
--rz-input-font-size: 20px;
}
To discover the class names and CSS variables used by a component right click the element in your browser and choose "Inspect" (or "Inspect element" depending on the browser). Then you can examine the HTML DOM tree: