Hi Ishan,

Sorry for this issue you are facing.

Style sheets can be created in different ways.

You can create a style sheet which works specifically for one component and also you can create a stylesheet which you can use across multiple lightning components and applications.

If you need to use your style sheet to multiple lightning components or lighting application, then it’s always recommended to define a common style sheet in a separate file with extension as .css. Upload this external CSS file as a static resource and it will be included in Lightning component using
<ltng:require styles=”{! $Resource.staticResourceFileName}”/>  tag.

Sample CSS file is below:

demoCss.css (external CSS file):

/*external css file with .css file extension */
 p{
     font-size:20px;
     color:red;
     background-color:black;
   }
  
  .divStyle{
       background-color:lightgreen;
       color:white;
       padding:20px; 
     }

YourComponent.cmp:

<aura:component>
<ltng:require styles="{!$Resource.demoCss}"/>
    <div class="slds-p-bottom_large slds-p-left_large" style="width:500px">
        <lightning:recordEditForm aura:id="recordViewForm"
                                     recordId="003R00000000000000"
                                     recordTypeId="012R00000000000000"
                                     objectApiName="Contact">
            <lightning:messages />
            <lightning:inputField fieldName="FirstName" />
            <lightning:inputField fieldName="LastName" />
            <lightning:inputField fieldName="Birthdate" />
            <lightning:inputField fieldName="Phone" />
            <!--Picklist-->
            <lightning:inputField fieldName="Level__c" />
            <lightning:button aura:id="submit" type="submit" label="Update record" class="slds-m-top_medium" />
            </lightning:recordEditForm>
</div>
</aura:component>

• http://sfdcmonkey.com/2017/02/06/use-css-lightning-component/