::file-selector-button

input {
  margin-top: 1rem;
}

input::file-selector-button {
  font-weight: bold;
  color: dodgerblue;
  padding: 0.5em;
  border: thin solid grey;
  border-radius: 3px;
}

<label for="avatar">プロフィール画像を選択:</label><br />

<input id="avatar" type="file" name="avatar" accept="image/png, image/jpeg" />

::file-selector-button {
  /* ... */
}

<form>
  <label for="fileUpload">ファイルをアップロードしてください</label>
  <input type="file" id="fileUpload" />
</form>

form {
  display: flex;
  gap: 1em;
  align-items: center;
}

input[type="file"]::file-selector-button {
  border: 2px solid #6c5ce7;
  padding: 0.2em 0.4em;
  border-radius: 0.2em;
  background-color: #a29bfe;
  transition: 1s;
}

input[type="file"]::file-selector-button:hover {
  background-color: #81ecec;
  border: 2px solid #00cec9;
}

<form>
  <label for="fileUpload">ファイルをアップロードしてください</label>
  <input type="file" id="fileUpload" />
</form>

form {
  display: flex;
  gap: 1em;
  align-items: center;
}

input[type="file"]::file-selector-button {
  border: 2px solid #6c5ce7;
  padding: 0.2em 0.4em;
  border-radius: 0.2em;
  background-color: #a29bfe;
  transition: 1s;
}

input[type="file"]::-ms-browse:hover {
  background-color: #81ecec;
  border: 2px solid #00cec9;
}

input[type="file"]::-webkit-file-upload-button:hover {
  background-color: #81ecec;
  border: 2px solid #00cec9;
}

input[type="file"]::file-selector-button:hover {
  background-color: #81ecec;
  border: 2px solid #00cec9;
}