To style your Visualforce page to match the Lightning Experience UI when viewed in Lightning Experience or the Salesforce1 app, set lightningStylesheets="true" in the <apex:page> tag. When the page is viewed in Salesforce Classic, it doesn’t get Lightning Experience styling.
Here is a standard Visualforce page without the lightningStylesheets attribute. The page is styled with the Classic UI.
Here is the same Visualforce page with the lightningStylesheets attribute set to true.
You can style most commonly used Visualforce components with the lightningStylesheets attribute. However, some components differ slightly in style from Lightning Experience. For example, <apex:selectOptions>, <apex:selectRadio>, <apex:inputFile>, and some <apex:inputField> elements use the browser’s default styling instead. Commonly used Visualforce components that don’t require styling, such as <apex:form>, <apex:outputText>, and <apex:param>, are still supported.