Boonex allows to edit the styles of this button.
Into the template base, there is a file called "swf_upload_integration.html", so you must have a copy of this file also for your template (you can copy the orginal file of the template base into your template root)
Open this file and search this code part:
/*window.onload = function()*/
    function bxInitSwfUploader() {
        bx_swf_uploader = new SWFUpload({
            // Backend Settings
            upload_url: "__accept_file__",
            post_params : {"action": "accept_multi_files", "oid": "__owner_id__" __possible_album__ },
            // File Upload Settings
            file_size_limit : "__file_size_limit__",
            file_types : "__accept_format__",
            file_types_description : "__accept_format_desc__",
            file_upload_limit : "__file_upload_limit__",
            file_queue_limit : "__file_queue_limit__",
            // Event Handler Settings (all my handlers are in the Handler.js file)
            file_dialog_start_handler : fileDialogStart,
            file_queued_handler : fileQueued,
            file_queue_error_handler : fileQueueError,
            file_dialog_complete_handler : fileDialogComplete,
            upload_start_handler : uploadStart,
            upload_progress_handler : uploadProgress,
            upload_error_handler : uploadError,
            upload_success_handler : uploadSuccess,
            upload_complete_handler : uploadComplete,
            // Button Settings
            //ORIGINAL LINE: 
            //button_window_mode: "__button_wmode__",
            //the next line allow to get a transparent button
            button_window_mode: SWFUpload.WINDOW_MODE.TRANSPARENT,
            //button_image_url : "__button_image_url__",
            button_image_url : "__button_image_url__",
            button_placeholder_id : "spanButtonPlaceholder",
            button_width: 136,
            //ORIGINAL VALUE 28px, I'VE CHANGED THE BOTTON HEIGHT TO 26
            button_height: 26,
            //ORIGINAL VALUE 3 : 6, I'VE CHANGED IT TO 3 : 5. This value allows to set the padding top for the text
            button_text_top_padding : $.browser.msie ? 3 : 5, 
            button_text : "<span class="swf_center">__Upload_lbl__</span>",
            //ORIGINAL LINE:
            //button_text_style : ".swf_center { text-align:center;font-family:'Helvetica Neue',Helvetica,Arial,Verdana,sans-serif;color:#333;font-size:12px;font-weight:bold;text-shadow:0 1px 0 #FFFFFF;}", 
            //I've chosen the color red (#ff0000) for the button text
            button_text_style : ".swf_center { text-align:center;font-family:'Helvetica Neue',Helvetica,Arial,Verdana,sans-serif;color:#333;font-size:12px;font-weight:bold;text-shadow:0 1px 0 #FF0000;}", 
            
            // Flash Settings
            flash_url : "__plugins_url__swfupload/swf/swfupload.swf",
/*
            swfupload_element_id : "flashUI2",        // Setting from graceful degradation plugin
            degraded_element_id : "degradedUI2",    // Setting from graceful degradation plugin
*/
            custom_settings : {
                progressTarget : "fsUploadProgress",
                cancelButtonId : "btnCancel"
            },
            // Debug Settings
            debug: false
        });
     }
Also, you can edit the background color of the button as you want. For this you must open the file "button_sprite.png" of your custom template in order to tune the design in according with the size (height and width) you have specified on the file "swf_upload_integration.html".
We hope this can help.
Regards
