It is possible to access and change the styling of inner elements of standard component with static resource - as shown in answer by @BlackCat
I see that you have created component
various for testing this. If you try to inspect the element created:
<div c-various_various="" class="row slds-m-vertical_large">
<h2 c-various_various="" class="header">Basic Examples</h2>
<lightning-textarea c-various_various="" class="mytextarea slds-form-element"><label for="input-3"
class="slds-form-element__label">Enter some text</label>
<div class="slds-form-element__control slds-grow"><textarea id="input-3" name="input1"
Here, as you see the class
mytextarea is added to the wrapper component. So, even if you increase the height, the styles/height of inner
textarea element will not change. The height will be applied to outer wrapper element only. So, when you modify the styles like color, background-color etc, it will be applied to wrapper element.
Also note that you cannot do something like
because thats inside closed shadow DOM. This was possible earlier with Aura component, but now its not possible because even Aura is using LWC component behind the scenes (according to unofficial salesforce support)