refactor: deduplicate CSS parser helpers and drop redundant base test

- Collapse extractRootBlock + extractDarkBlock into a single extractBlock
  helper; the two functions shared identical body logic (same regex, same
  return) and differed only in the string they received — now the call
  sites pass the appropriate slice inline.
- Remove the second test in base.test.ts ("covers all three form control
  selectors in one rule"): the first test's regex already asserts all
  three selectors appear together with font-size: 16px, so the individual
  /input/, /textarea/, /select/ matches added no coverage.

tests/base.test.ts

@@ -8,9 +8,4 @@ describe('base.css', () => {
     expect(css).toMatch(/input\s*,\s*textarea\s*,\s*select\s*\{[^}]*font-size\s*:\s*16px/);
   });
 
-  it('covers all three form control selectors in one rule', () => {
-    expect(css).toMatch(/input/);
-    expect(css).toMatch(/textarea/);
-    expect(css).toMatch(/select/);
-  });
 });