fix(forms): modernize CSS themes with :where() zero-specificity reset

[faisalahammad]

Summary

Replace the 5 opinionated color themes (light/dark/red/blue/green) with a minimal :where() CSS reset that uses zero specificity — themes and plugins always override without fighting specificity wars. Reduces CSS from ~320 lines to ~48.

Fixes #844

Changes

assets/src/css/form-basic.css

Before:

/* ~122 lines with high-specificity selectors */
.mc4wp-form { ... }
.mc4wp-form label { ... }
.mc4wp-form input[type="text"] { ... }

After:

/* ~48 lines with zero-specificity :where() selectors */
:where(.mc4wp-form) { margin: 1em 0; }
:where(.mc4wp-form label) { display: block; font-weight: bold; margin-bottom: 6px; }
:where(.mc4wp-form input[type="submit"]) { cursor: pointer; display: inline-block; appearance: none; }
/* Alert colors stay at normal specificity (functional) */
.mc4wp-alert { color: #c09853; }
.mc4wp-error { color: #cd5c5c; }

Why: :where() always has specificity 0, so theme CSS wins without !important. Honeypot hiding uses normal specificity (must not be overridable). CSS logical properties (margin-inline-end) replace directional rules for native RTL support.

assets/src/css/form-themes.css

Deprecated — contains only a comment. Minified output is 0 bytes.

includes/admin/migrations/5.0.0-modernize-css.php

New file. Converts all mc4wp-form posts with theme-* CSS setting to basic, rebuilds mc4wp_form_stylesheets option without themes.

Why: Existing forms using color themes need migration to avoid broken references.

includes/forms/class-form.php

Before:

public function get_stylesheet() {
    $stylesheet = $this->settings['css'];
    if (empty($stylesheet)) { return ''; }
    return $stylesheet;  // theme-* returned as-is → no CSS loads
}

After:

public function get_stylesheet() {
    $stylesheet = $this->settings['css'];
    if (empty($stylesheet)) { return ''; }
    if (strpos($stylesheet, 'theme-') === 0) { return 'basic'; }  // safety net
    return $stylesheet;
}

Why: Un-migrated forms (e.g. fresh zip install without running migration) still work — theme-* values silently fall back to basic.

includes/forms/class-form-element.php

Removed mc4wp-form-theme class generation block.

includes/forms/class-asset-manager.php

Removed 'themes' from get_registered_stylesheets() — now returns only ['basic'].

includes/forms/views/tabs/form-appearance.php

Removed 5 color theme <option> entries from the CSS dropdown. Only "Inherit" and "Basic" remain. mc4wp_admin_form_css_options filter preserved for third-party use.

tests/FormTest.php

Added assertions for theme-* → basic fallback and custom stylesheet passthrough.

Testing

Test 1: Basic CSS renders correctly

1. Create/edit form → Appearance tab → set CSS to "Basic (minimal reset)"
2. View form on frontend

Result: Inputs layout correctly, no forced colors/borders.

Test 2: Inherit option works

1. Set form CSS to "Inherit from [theme]"
2. View on frontend

Result: Form uses active WP theme styles.

Test 3: Existing theme- forms migrated*

1. Install plugin on site with forms using color themes
2. Check form editor → Appearance tab

Result: "Basic" selected. Frontend renders with basic reset.

Test 4: Admin UI simplified

1. Form editor → Appearance tab

Result: Dropdown shows only "Inherit" and "Basic" options.

Test 5: RTL checkbox spacing

1. View form with checkboxes in RTL language (Arabic)

Result: Checkbox margin on correct side (margin-inline-end).