feat(react,elements): white-label theming API for the checkout widget

[aguilar1x]

Context

The checkout widget shipped in the first campaign (#15, #16) is visually generic. Merchants integrating a payment widget into their own product expect to match their brand — without that, adoption stalls at the demo stage.

Scope

• A theming API (CSS custom properties / design tokens) covering colors, typography, border-radius, and spacing, consumable by both connect-react and connect-elements.
• Logo/brand-mark injection into the checkout header.
• Locale/i18n support for widget copy (starting with en/es, matching Pacto's SINPE/SPEI regional focus).

Acceptance criteria

• A merchant can restyle the widget to match their brand using only documented CSS variables, no fork required.
• The same theme config works identically in the React component and the web-component build.
• Widget copy renders correctly in both supported locales, with a documented path to add more.

[Kend07]

Hello maintainer, I have reviewed the issue and I am ready to work on it. I have experience with CSS, HTML, Typescript and PplgSQL, Let me help you!
I participated in the previous GranfFox Campaign (First Ofifcial Campaign) I will take over and deliver a HQ PR.

[Ayinkx01]

Hi l would like to take this job

[felipevega2x]

Hey, I’m Felipe Vega

Computer Science student with 2+ years of experience building full-stack applications through personal and collaborative projects. Currently working at Intel and focused on AI development, Node.js, and Next.js.

[grantfox-oss]

🦊 GrantFox — @felipevega2x has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #45)
2. Your PR will be reviewed by the PACTO-LAT maintainers

Good luck! Track your progress on GrantFox.