Scope the theme's close button rules to the theme's own close button

[nlemoine]

Two bugs, same cause, so I put them in one PR.

Alert renders its dismiss button through Button, so it comes out as btn btn-invisible btn-close. The theme has five rules written for that button, in buttons.css and alerts.css. All five are selected on .btn-close alone. So they also hit plain Bootstrap close buttons: modal headers, toasts, offcanvas. Those don't have the --button-* custom properties (only the .btn rule declares them), and they don't have the .btn layout that some of these rules exist to undo.

Blank on hover, buttons.css:

.btn-close:not(:disabled):not(.disabled):hover {
    background: var(--button-hover-bg, var(--button-bg));
}

Both properties are missing on a plain close button. The declaration is then invalid at computed-value time, background falls back to its initial longhands, and background-image: none wipes the glyph. Bootstrap draws that glyph as a data-URI background, so the button is just empty on hover.

Black glyph in dark mode, alerts.css:

[data-bs-theme="dark"] .btn-close { filter: none }

Fine for our own button, its glyph is a child <svg> in currentColor, inverting it would be wrong. On a plain one it cancels --bs-btn-close-filter, and that filter is what turns Bootstrap's black glyph white. Black glyph on a dark background.

Repro: drop <button type="button" class="btn-close" data-bs-dismiss="modal"></button> into a modal header on any EasyAdmin page. Hover it, glyph gone. Switch to dark, glyph black.

The fix scopes the five rules to .btn.btn-close, so plain close buttons are left to Bootstrap. I also switched the two background declarations to background-color. My guess is the color was the intent anyway, the transition-property right above says background-color. The shorthand would keep wiping the glyph even where the variables resolve, it just doesn't show there.

Assets rebuilt with make build-assets. Compiled diff in public/app.*.css is those five rules and nothing else.