some websites help you to know this for ex:wpthemedetector.com
another way is to refer to style.css
in your case this was in style.css
Author URI: https://www.themeisle.com
Description: Constructzine Lite is a free construction company Business WordPress Theme, responsive, high-resolution corporate theme built with construction companies / businesses in mind, featuring a pixel perfect responsive corporate design, large images, other features include optional social menu, sidebar,clean, modern and minimal responsive design footer widgets and blog / business template.Some other businesses that uses it are : translators, interior design, architec, plumber,lawyer, painter, roofing business,repairing or even it consulting ones.All options are using WordPress Theme Customizer feature so you can watch changes live! Official support forum (https://themeisle.com/forums/forum/constructzine-lite).
Version: 1.3.1
Tags: yellow, gray,blue, black, white, dark, light, two-columns,one-column, right-sidebar,full-width-template,fixed-layout, custom-menu, featured-images, sticky-post, translation-ready, responsive-layout, theme-options,threaded-comments, custom-header, rtl-language-support, custom-background,front-page-post-form
License: GNU General Public License version 2
License URI: license.txt
Constructzine Lite is distributed under the terms of the GNU GPL
*/
when one change the theme there is not any way to know what theme is used.
in these cases you just know a theme if you know it before!!
best regards
Rohallah
