read this but please telling me how to do ( modify which file ? )
https://www.mediawiki.org/wiki/User:CoolCornucopia#Tips:_Change_collapse/expand/empty_images_in_the_CategoryTree
User:CoolCornucopia#Tips: Change collapse/expand/empty images in the CategoryTree
Tips: Change collapse/expand/empty images in the CategoryTree
MediaWiki:categorytree-collapse-bullet
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10 <![if !IE]>
11 <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQBAMAAADt3eJSAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAA9QTFRFeXl53d3dmpqasbG
12 x////GU0iEgAAAAV0Uk5T/////wD7tg5TAAAAK0lEQVQI12NwgQIG0hhCDAwMTCJAhqMCA4MiWEoIJABiOCooQhULi5BqMgB2bh4svs8t+QAAAABJRU5ErkJggg==" width="16" height="16""/>
13 <![endif]>
MediaWiki:categorytree-expand-bullet
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11 <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAAxQTFRF3d3deXl5////nZ2
12 dQA6SoAAAAAN0Uk5T//8A18oNQQAAADNJREFUeNpiYEIDDMQKMKALMDOgCTDCRWACcBG4AEwEIcDITEAFuhnotmC4g4EEzwEEGAADqgHmQSPJKgAAAABJRU5ErkJggg==" width="16" height="16"/>
13 <![endif]>
MediaWiki:categorytree-empty-bullet
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11 <![if !IE]>
12 <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAgMAAABinRfyAAAAAXNSR0IArs4c6QAAAAxQTFRFeXl53d3dmpqasbGxwe9qYQAAAAF0Uk5TAE
13 Dm2GYAAAABYktHRACIBR1IAAAACXBIWXMAAAsTAAALEwEAmpwYAAAAB3RJTUUH3AQaDSEvTJrcJgAAA
14 AxJREFUCNdjYKAuAAAAUAABIhPodQAAAABJRU5ErkJggg==" alt="[x]" width="16" height="16"/>
15 <![endif]>