Trails has complete customizability on theming the widget to brand it to your application by simply passing your desired css to the customCss variable: 
import { TrailsWidget } from '0xtrails/widget'

export const App = () => {
  return (
    <TrailsWidget
      mode="pay"
      theme="light"
      customCss={`--trails-font-family: "Trebuchet MS", "Arial", sans-serif;
--trails-border-radius-widget: 32px;
--trails-border-radius-button: 24px;
--trails-border-radius-input: 24px;
`}
    >
      <button className="custom-button-styles">
        Pay with Trails
      </button>
    </TrailsWidget>
  )
}
Here’s the full list of configurability for the possible css variables to change: 
--trails-font-family: "Trebuchet MS", "Arial", sans-serif;
--trails-border-radius-widget: 32px;
--trails-border-radius-button: 24px;
--trails-border-radius-input: 24px;
--trails-border-radius-dropdown: 12px;
--trails-border-radius-container: 8px;
--trails-border-radius-list: 8px;
--trails-widget-border: 4px solid rgb(0 82 204);
--trails-primary: rgb(0 82 204);
--trails-primary-hover: rgb(0 66 163);
--trails-primary-disabled: rgb(209 213 219);
--trails-primary-disabled-text: rgb(107 114 128);
--trails-bg-primary: rgb(255 255 255);
--trails-bg-secondary: rgb(248 250 252);
--trails-bg-tertiary: rgb(241 245 249);
--trails-bg-card: rgb(255 255 255);
--trails-bg-overlay: rgb(255 255 255);
--trails-text-primary: rgb(15 23 42);
--trails-text-secondary: rgb(0 82 204);
--trails-text-tertiary: rgb(51 65 85);
--trails-text-muted: rgb(148 163 184);
--trails-text-inverse: rgb(255 255 255);
--trails-border-primary: rgb(0 82 204);
--trails-border-secondary: rgb(226 232 240);
--trails-border-tertiary: rgb(241 245 249);
--trails-hover-bg: rgb(248 250 252);
--trails-hover-text: rgb(15 23 42);
--trails-focus-ring: rgb(0 82 204);
--trails-success-bg: rgb(240 253 244);
--trails-success-text: rgb(22 163 74);
--trails-success-border: rgb(187 247 208);
--trails-warning-bg: rgb(255 251 235);
--trails-warning-text: rgb(217 119 6);
--trails-warning-border: rgb(253 230 138);
--trails-error-bg: rgb(254 242 242);
--trails-error-text: rgb(220 38 38);
--trails-error-border: rgb(254 202 202);
--trails-shadow: 0 1px 3px 0 rgb(0 0 0 / 0.1), 0 1px 2px -1px rgb(0 0 0 / 0.1);
--trails-input-bg: rgb(255 255 255);
--trails-input-border: rgb(226 232 240);
--trails-input-text: rgb(15 23 42);
--trails-input-placeholder: rgb(148 163 184);
--trails-input-focus-border: rgb(0 82 204);
--trails-input-focus-ring: rgb(0 82 204);
--trails-dropdown-bg: rgb(255 255 255);
--trails-dropdown-border: rgb(226 232 240);
--trails-dropdown-text: rgb(15 23 42);
--trails-dropdown-hover-bg: rgb(248 250 252);
--trails-dropdown-selected-bg: rgb(241 245 249);
--trails-dropdown-selected-text: rgb(15 23 42);
--trails-dropdown-focus-border: rgb(0 82 204);
--trails-list-bg: rgb(255 255 255);
--trails-list-border: rgb(226 232 240);
--trails-list-hover-bg: rgb(240 248 255);