[FormGroup] add contentClassName and style props (#3120)

* add contentClassName prop

* add style prop

packages/core/src/components/forms/formGroup.tsx

@@ -10,6 +10,12 @@ import * as Classes from "../../common/classes";
 import { DISPLAYNAME_PREFIX, IIntentProps, IProps } from "../../common/props";
 
 export interface IFormGroupProps extends IIntentProps, IProps {
+    /**
+     * A space-delimited list of class names to pass along to the
+     * `Classes.FORM_CONTENT` element that contains `children`.
+     */
+    contentClassName?: string;
+
     /**
      * Whether form group should appear as non-interactive.
      * Remember that `input` elements must be disabled separately.
@@ -39,21 +45,24 @@ export interface IFormGroupProps extends IIntentProps, IProps {
      * Optional secondary text that appears after the label.
      */
     labelInfo?: React.ReactNode;
+
+    /** CSS properties to apply to the root element. */
+    style?: React.CSSProperties;
 }
 
 export class FormGroup extends React.PureComponent<IFormGroupProps, {}> {
     public static displayName = `${DISPLAYNAME_PREFIX}.FormGroup`;
 
     public render() {
-        const { children, helperText, label, labelFor, labelInfo } = this.props;
+        const { children, contentClassName, helperText, label, labelFor, labelInfo, style } = this.props;
         return (
-            <div className={this.getClassName()}>
+            <div className={this.getClassName()} style={style}>
                 {label && (
                     <label className={Classes.LABEL} htmlFor={labelFor}>
                         {label} <span className={Classes.TEXT_MUTED}>{labelInfo}</span>
                     </label>
                 )}
-                <div className={Classes.FORM_CONTENT}>
+                <div className={classNames(Classes.FORM_CONTENT, contentClassName)}>
                     {children}
                     {helperText && <div className={Classes.FORM_HELPER_TEXT}>{helperText}</div>}
                 </div>