Just add the additional parameter " nocss: true " when loading Google Custom Search so that Google does not load css for search, so you can define all classes gcse-, gs-, etc. without interference.
nocss: a boolean that tells the API whether to load stylesheets typically associated with its controls. If you are not going to use CSS by default, you can reduce the load time by setting this to true. the default value is false.
https://developers.google.com/loader/#DetailedDocumentation
The code:
google.load('search', '1', { callback: OnGoogleSearchLoad, language : 'en', nocss: true, style : src="http://example.com/assets/css/gcse.css" });
Result (no css, except for the default rules for the site):

Example css file for GCS:
.gsc-tabsAreaInvisible, .gsc-resultsHeader, .gsc-branding, .gcsc-branding, .gsc-url-top, .gs-watermark, .gsc-thumbnail-inside, .gsc-url-bottom { display: none; } .gsc-result { padding: 20px 0; border-bottom: 1px solid #E1E1E1; } .gs-image-box { width: 140px; height: 80px; overflow: hidden; } img.gs-image { min-height: 80px; } td.gsc-table-cell-thumbnail { vertical-align: top; padding: 0; width: 140px; display: block!important; } td.gsc-table-cell-snippet-close { vertical-align: top; padding: 0; padding-left: 20px; } .gsc-wrapper { font-size: 16px; line-height: 20px; } .gsc-control-cse a { color: #202020; font-family: HelveticaNeueLTPro-Cn, Helvetica, Arial, sans-serif; } .gs-snippet { color: #777; margin-top: 10px; } b { font-weight: normal; } .gsc-cursor { width: 100%; height: 20px; text-align: center; } .gsc-cursor-page { display: inline-block; width: 20px; height: 20px; cursor: pointer; text-align: center; margin: 20px 0; } form.gsc-search-box { background: #d9dadd; border: 20px solid #d9dadd; width: 100%; -webkit-box-sizing: border-box; -moz-box-sizing: border-box; box-sizing: border-box; } table.gsc-search-box { width: 100%; } td.gsc-search-button { vertical-align: middle; padding-left: 20px; } td.gsc-input { vertical-align: top; width: 100%; } input.gsc-input { margin:0; width: 99%; }
Result (with custom styles):

Gniewomir Świechowski
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