Happyforms

Yes you can do that already without having to add additional class names.

All HTML elements have class names already. Target those classes to create your own styles with CSS.

Here are the primary static class names from the provided Happyforms HTML structure. These appear consistent and non-dynamic (avoiding IDs with numbers like “3006” or generated option IDs), making them ideal for CSS targeting.
• Container classes: happyforms-flex, happyforms-form__part
• Core part classes: happyforms-part, happyforms-part-wrap
• Label classes: happyforms-part__label-container, happyforms-part__label, label, happyforms-required
• Input/Element classes: happyforms-part__el, happyforms-input
• Field-specific classes: happyforms-part–single_line_text, happyforms-part–email, happyforms-part–multi_line_text, happyforms-part–checkbox, happyforms-part–select, happyforms-part–payments
• Checkbox/Option classes: happyforms-part__option, happyforms-part-option, option-label, checkmark
• Select classes: happyforms-custom-select, happyforms-part__select-wrap, happyforms-select, happyforms-placeholder-option
• Payments classes: happyforms-payments__price, happyforms-payments-service, happyforms-payments__stripe-el, happyforms-payments__card, stripe-element
• Submit/Button classes: happyforms-submit, happyforms-button–submit
• Utility classes: happyforms-part-error-notice, screen-reader-text, happyforms-visuallyhidden
CSS Targeting Example
Use these for custom styles, e.g.:

.happyforms-part__label { font-weight: bold; color: #333; } /* Labels /
.happyforms-input input { border: 2px solid #007cba; padding: 12px; } / Text inputs /
.happyforms-part–payments .price { font-size: 1.5em; color: green; } / Price display */
This gives complete control without conflicting with dynamic elements. Focus on width modifiers like happyforms-part–width-half or happyforms-part–width-full for layout tweaks.

This will be changed in the following update.