Add the attribute LightningStylesheet and Salesforce get the work done for you.
Here is a sample page I made to illustrate the changes in Salesforce Classic and Lightning Experience.
|Salesforce Classic with Inputs|
|Lightning Experience with Inputs|
- This is a beta version of the feature. It is still under development by the Salesforce team.
- You can also use the apex:slds tag for creating custom component
- It is only available in the Winter 18 api (41). You will have this error with older api version: "Unsupported attribute lightningstylesheets in <apex:page>"