HI!

1. You need to edit: wp-content/themes/thegem/css/thegem-preloader.css 

Or use next css:

body #page-preloader {    background: #2c2e3d;
}
body #page-preloader .page-preloader-spin,
body .block-preloader .page-preloader-spin,
body .preloader-spin {    border: 2px solid transparent;    border-top-color: #b9b7ff;
}
body .preloader-spin {    border-top-color: #b9b7ff;
}
body #page-preloader .page-preloader-spin:before,
body .block-preloader .page-preloader-spin:before,
body .preloader-spin:before {    border: 2px solid transparent;    border-top-color: #00bcd4;
}
body .preloader-spin:before {    border-top-color: #00bcd4;
}
body #page-preloader .page-preloader-spin:after,
body .block-preloader .page-preloader-spin:after,
body .preloader-spin:after {    border: 2px solid transparent;    border-top-color: #a3e7f0;
}
body .preloader-spin:after {    border-top-color: #7b97ab;
}

In order to change loader to your image, use this css:

#page-preloader.preloader-loaded .page-preloader-spin {    background-image: URL('URL_OF_THE_IMAGE');    background-size: cover;    width: 150px;    height: 150px;    border: none;
}
#page-preloader.preloader-loaded .page-preloader-spin:before
#page-preloader.preloader-loaded .page-preloader-spin:after {    display: none;    }

2.  We recommend to use Loco Translate plugin for the theme translation or you can translate theme manually with .po file, pls. follow the instructions: 

1. Take file languages/thegem.pot or xx_YY.po from theme directory.

2. Open this file with Poedit software.

3. Translate necessary strings to your language.

4. In software open Catalog - Settings and set Language to Translation to xx_YY.

5. Then save the file as thegem-xx_YY. (You must have two files thegem-xx_YY.po and thegem-xx_YY.mo.)

6. These files you need to put to the wp-content/languages/themes of your site.

With the same operation, you can translate TheGem elements plugin.

Just put  translations in wp-content/languages/plugins

Welcome!