Basic form styles are give below - field comes with three sizes. By default all form fields are 100% width - so it's depends on container size. Form input's have default style, hover and focus style.
<div class="form-field form-field-search small">
<span class="form-field-search-icon"><i class="icon icon-search icon-small"></i></span>
<input type="text" name="" class="form-input" placeholder="Search ...">
</div>
<div class="form-field form-field-search">
<span class="form-field-search-icon"><i class="icon icon-search"></i></span>
<input type="text" name="" class="form-input" placeholder="Search ...">
</div>
<div class="form-field form-field-search large">
<span class="form-field-search-icon"><i class="icon icon-search"></i></span>
<input type="text" name="" class="form-input" placeholder="Search ...">
</div>
<div class="form-field form-field-search loading">
<i class="form-field-search-icon ion-search"></i>
<input type="text" name="" class="form-input" placeholder="Search ...">
<i class="form-field-search-loading-icon"></i>
</div>
<div class="form-field form-field-search loading hint">
<i class="form-field-search-icon ion-search"></i>
<input type="text" name="" class="form-input" placeholder="Search ...">
<i class="form-field-search-loading-icon"></i>
<div class="form-field-search-hint">
<ul>
<li><a href="#">Afganistan</a></li>
<li><a href="#">Srilanka <span class="badge badge-info">new</span></a></li>
<li><a href="#">Bangladesh</a></li>
<li><a href="#">United state</a></li>
</ul>
</div>
</div>
If you're looking for tag implementation, then see this.
Reference from _form.scss
$form-input-radius: $global-border-radius !default;
:root {
--r-form-input-placeholder-color: var(--r-color-light-lighten-30);
--r-form-input-background-color: var(--r-color-dark);
--r-form-input-color: var(--r-color-white);
--r-form-input-border-color: var(--r-color-dark-lighten-5);
--r-form-input-hover-border-color: var(--r-color-link);
--r-form-input-focus-border-color: var(--r-color-primary);
--r-form-text-color: var(--r-color-light-lighten-15);
--r-form-checkbox-label-color: var(--r-color-dark-lighten-20);
--r-form-disable-input-color: var(--r-color-light-lighten-5);
--r-form-primary-color: var(--r-color-light-lighten-2);
--r-form-primary-border-color: var(--r-color-light-lighten-5);
--r-form-primary-hover-color: var(--r-color-info);
--r-form-primary-focus-color: var(--r-color-white);
--r-form-error-color: var(--r-color-danger);
--r-form-success-color: var(--r-color-success);
--r-form-warning-color: var(--r-color-info);
}