Hello Asad Ur Rehman !
The selected caption can be corrected as language key "_sys_txt_splash_join" (edit it via Polyglot app as other texts on this page). The styles of the Splash page can be edited via Studio->Designer->Splash page area. See the <style> section.
hi LeonidS, which line in <style> section to be edited to increrase the width of the Join Box/ Form, Moreover, to remove its background. M sorry i am not a developer, thnx