Fix#2387. Fix#2075. There was substantial refactoring of Writer Xlsx styles in 18.0. An existing static property `$theme` was intended to be shared by both Writer Xlsx and the new Writer Xlsx Styles. However, the initialization of the property in the latter happened later than it should have. This PR makes that initialization happen as soon as the theme has been read. Also, declaring that property as static seems questionable; I have made it an instance member. This small re-factoring makes it possible to now support Themes in tab colors.
Since this PR changes Reader/Xlsx/Styles, add type-hinting throughout that module to eliminate Phpstan/Scrutinizer problems. I also removed method readStyle from Reader/Xlsx, since it was essentially duplicated in Reader/Xlsx/Styles. And I added a small number of tests to ensure that Styles is 100% covered. All of this is necessary in preparation for Namespacing phase 2.
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